The quality of boar semen may fluctuate significantly due to environmental and endogenous influences (age, season, climate, genetics, nutrition, breed etc.). Knowledge of the physiological features of the sperm and seminal plasma and the impact of the breed, lines and individual features of the boar, are a prerequisite for the successful improvement of reproductive efficiency and the heredity of the best genetic traits of semen. Oxidative stress is the primary cause of fertility disorders in mammals, but semen has an antioxidative system which helps it to neutralize the effect of the production of excessive reactive oxygen compounds. Sperm is particularly susceptible to oxidative damage due to the excessive production of reactive oxygen compunds, and because of the large proportion of unsaturated fatty acids in the plasma membrane and a relatively low concentration of antioxidant enzymes in the cytoplasm. Porcine semen is composed of heterogeneous subpopulations of sperm that are distinguished by their functional and structural features, and the share of each subpopulation is associated with the ability of fertilization and the storing of semen. The aim of the study was to determine the influence of boar breeds and hybrids on semen quality, the parameters of antioxidant protection of both spermatozoa and seminal plasma of boars and biochemical parameters in their seminal plasma. Also, the aim was to investigate antioxidant parameters in different sperm fractions, separated in iodixanol density gradient solution. Semen samples of 27 boars (belonging to either breeds of Swedish Landrace, German Landrace, Large White, Pietrain or PIC-hybrid) aged between 1.5 and 3 years were collected by gloved-hand technique, once, in autumn season. After the evaluation of semen quality, the seminal plasma was separated from the sperm by centrifugation of semen (1,000 x g/15 min). Part of the native semen was centrifuged in a discontinuous iodixanol density gradient solution for 20 min at 1500 x g at 20°C. After the centrifugation, three sperm fractions were obtained: sperm with progressive mobility greater than 90%, sperm with progressive mobility less than 20% and abnormal sperm. In the seminal plasma the biochemical parameters as well as the concentration of total antioxidant status (TAS), and the activities of total superoxide dismutase (TSOD) and glutathione peroxidase (GSH-Px) were determined. Besides of the aforementioned parameters, in the washed sperm and and in three fractions of sperm the activities of copper-zinc superoxide dismutase and manganese superoxide dismutase were determined too. The best overall antioxidant protection in seminal plasma was determined in the PIC-hybrids, and the lowest in the Pietrain breed, while the overall antioxidant protection in the washed sperm was the lowest in the PIC-hybrids and the highest in Swedish Landrace. From the obtained results, it can be concluded that there are differences in the antioxidative protection of seminal plasma and sperm in the above mentioned boar breeds and PIC-hybrids. Diminished antioxidant protection of sperm in the PIC-hybrids could be compensated by a higher antioxidant protection of seminal plasma. In the abnormal sperm fraction, the concentration of TAS, and the activities of TSOD and GSH-Px were significantly higher (P<0.001) than those in the fractions of high and very low mobility of sperm. The lowest values of TAS and TSOD and GSH-PX were obtained in sperm with very low mobility (P<0.001). The results indicated a decrease in antioxidative protection of sperm with very low mobility in relation to the sperm with high mobility. The highest levels of TAS, SOD and GSH-Px found in abnormal sperm could indicate the enhanced antioxidative defence due to excessive production of reactive oxygen species in that sperm fraction. The sperm fraction of high mobility and normal morphology could be applied in the assisted reproduction, but systematic study of the antioxidant parameters, as well as the evaluation of their values in the above mentioned sperm fractions may contribute to a better understanding of fertilizing properties and causes of infertility. PIC-hybrid boars had a significantly higher concentrations of total cholesterol, LDLcholesterol, triacylglycerols in seminal plasma, and German Landrace boars had a significantly lower concentration of the all above mentioned lipids as compared to PIChybrids. The Pietrain boars had a significantly lower concentration of triacylglycerols and total cholesterol in compared to the PIC-hybrids. The determination of biochemical parameters, particularly lipid substances and antioxidant parameters in boar seminal plasma, contributes to a better assessment of boar semen quality and a higher concentration of the above mentioned fatty substances enables successful storage and preservation of the frozen semen.