Introduction Milk and dary products produced from cows, sheep, goats and buffalos are important part of Mediterranean nutrition. However, production of cheese from mixed milk from different dairy animals is common habits in Mediterranean countries. There are examples of cheeses produced from mixed milk, which belong to the groups of fresh, soft, albumin, semi hard, hard cheeses, as well as, cheeses that belong to the group of pasta filata type. Lećevački cheese is pastoral and traditional cheese, which belong to the group of hard Dalmatian and coastal cheeses. In the past, this cheese was produced on the family farms of the area of Lećevica from full fat sheep's milk, cow's milk, or mixed milk, depending on the season and herd composition. Nowadays, it is produced from mixed sheep's and cow's milk. Milk composition of mixed milk depend on many different variability of milk, in fact on composition and physico-chemical properties of different milk types which are used in the production of the cheese. Hypothesis and aims of the research The hypothesis of the thesis are:  Higher ratio of sheep's milk in mixed milk during the production of Lećevački cheese would effect: o Higher yield o More intensive proteolysis, which would change the stage of casein particles and texture of the cheese during ripening.  Season, year and flock would effect chemical composition, particularly the ratio of nitrogen compounds, physical properties and hygienic characteristic of sheep milk of Dalmatian Pramenka. Therefore, the aim of this thesis was to determine the influence of ratio of sheep's milk in mixed milk on cheese yield, proteolytic changes and textural properties of Lećevački cheese during ripening, as well as, influence of season, year and flock on chemical composition, especially on the ratio of nitrogen compounds, physical properties and hygienic quality of sheep's milk of Dalmatian Pramenka. Materials and methods It was produced 15 batches of Lećevaćki cheese with different ratio of sheep's and cow's milk. Ratio of sheep's milk was as follow: from 35 % fo 39 %, from 40 % to 44 % and from 45 % to 50 %. Sample of cheeses were taken from the begining, 30., 60. and 90. days of ripening. Sheep's milks were obtained from six family farms in sub-Mediterranean area of Croatia where Dalmatian Pramenka is reared on natural pastures. All farms included in this research breed Dalmatian Pramenka and they were located in the hilly area of Dalmatian hinterland on the altitude between 200 m and 500 m. Milk was analysed by standard methods for physical and chemical properties. Cheese was analysed by standard method for physical and chemical properties. Cheese yield of Lećevački cheese was determined by VanSkyke formula. Proteolytic changes during ripening of Lećevaćki cheese were followed by standard methods that included four indicators of ripening as follow:  Σγ-CN/β-CN (total relative ratio of γ-CN in β-CN),  αs1I-CN/( αs1I-CN + αs1-CN) (relative ratio of degraded fragments of αs1-I-CN in sum of αs1-CN i αs1I-CN).  Percentage of water soluble nitrogen fraction in total nitrogen (WSN/TN)  Percentage of 12 % three clorine aceticacid soluble nitrogen fraction in total nitrogen (TCASN/TN). Textural properties were determine by the method of compression test by using textural analyser (TA Plus Lloyd Instruments, UK), which was equiped by 500 N mesearing unit, as a tool surounded probe was used with dyameter of 50 mm. Cheese samples were used through out whole cut surphace cheese zone by spetial auger. Analysis were perfomed by software program Nexygen plus from which figures following parameters were calculated:  Fracture stress (kPa)  Percentage strain at fracture  Elasticity (mm). Statistical analysis were conducted by using statistic program SAS (2001). For description of analysed variables, method of descriptive statistics were used (PROC UNIVARIATE). The effect of different ratio of sheep's milk in mixed milk on physicochemical compositon, proteolysis and textural properties of Lećevački cheese, as well as, the effect of season, year and flock on chemical composition, physical properties and hygienic qualty of milk of Dalmatian Pramenka were tested by analysis of variance (PROC GLM). Results and conclusions Higher ratio of sheep's milk (from 40 % to 44 %) increased ratio of fat, protein, casein, lactose and total solids non-fat and total solids, while decreased the ratio of nonprotein nitrogen in mixed milk. Ripening time significantly (P<0.01) increased the ratio of milk fat, protein and total dry metter of Lećevački cheese. Moreover, it was found that all ripening indicators (index alpha and beta, water soluble nitrogen and 12% three clorine aceticacid soluble nitrogen in total nitrogen) significantly (P<0.01) increased too. Simillarly, ripening time caused significantly (P<0.01) increasing of fracture stress (kPa), significant decreasing of percentage strain at break and elasticity (mm). Higher percentage of sheep's milk in mixed milk (from 45 % fo 50 %) caused significant (P<0.01) decreasing of total dry metter and milk fat content, as well as, significant (P<0.05) increasing of protein content in Lećevački cheese. Different ratio of sheep's milk in mixed milk did not effect any, previously mentioned, ripening indicators. Higher ratio of sheep's milk in mixed milk decreased actual yield (P = 0.11) and fat recovery (P = 0.074) in the production of Lećevački cheese. Moreover, higher ratio of sheep's milk in mixed milk increased fracture stress (kPa; P = 0.088) and significanlty decreased (P<0.05) percentage strain at break and elasticity (mm). Significant effects were observed for season (P<0.001) and flock (P<0.05) on all analyzed chemical components of milk. Significan effect of the year was observed on lactose, casein, non-protein nitrogen (NPN; P<0.01), protein (P<0.05) and somatic cell count (SCC) and bacterial count values (TB; P<0.05). Acording to the results of this investigations it could be concluded that it is not reasonlable to produce Lećevački cheese with sheep's milk in mixed milk in ratio higher then 39 % taking into consideration that higher ratio of sheep's milk in mixed milk decreased actual yield and fat recovery in the production of Lećevački cheese. Moreover, Lećevački cheese with higher ratio fo sheep's milk contained lower total dry metter and milk fat. Fracture stress (kPa), percentage strain at break and elasticity (mm) were higher as a consequence of decreasing of fat content and simultaneously increasing of protein content in Lećevački cheese.