Activity of oleuropein, a phenol secoiridoid present in olive tree products, its derivative hydroxytyrosol and water extract of olive leaf were investigated using in vitro tests against opportunistic fungal pathogen Candida albicans. Antifungal susceptibility testing was used to estimate the drop of viability up to 80 % in comparison to the control (untreated cells). Minimal inhibitory concentration (MIC) against C. albicans for oleuropein was 12,5 mg/ml, for hydroxytyrosol was 6,25 mg/ml and for water extract of olive leaf was 25 mg/ml. Morphological changes in the nuclei after staining with fluorescent DNA-binding dyes revealed that apoptosis was a primary mode of cell death in analyzed samples treated with sub-MIC concentrations of tested compounds. In order to understand mode of action of oleuropein and hydroxytyrosol, their effect on C. albicans virulence factors essential for development of infection in the host have been investigated. Both oleuropein and hydroxytyrosol modulate morphogenetic coversion and inhibit filamentation of C. albicans induced in the conditions of reduced nutrient availability. On the other hand, treatment with sub-MIC concentractions of these phenolic compounds did not affect C. albicans biofilm formation. The hydrophobicity assay showed that both compounds, in sub-MIC values, have decreased the cellular surface hydrophobicity, a factor associated with adhesion to epithelial cells. Cultivation of C. albicans in the presence of oleuropein and hydroxytyrosol also inhibits, in the dose-dependent manner, acitivity of aspartyl-proteases (Sap), hydrolytic enzymes important in adherence, tissue penetration, invasion and destruction of host tissue. Most therapies for fungal infections target the ergosterol biosynthesis pathway or its end product ergosterol which is necessary for growth and normal function of fungal cells. At subinhibitory concentrations tested compounds have altered sterol content and subsequently affected the cell membrane of C. albicans. Both compounds induced dose-dependent loss of intracellular material to outer space (DNA, protein leakage) and caused membrane depolarisation indicating their direct effect on the membrane. In addition, cytotoxicity and genotoxicity assays were performed to elucidate the effect of oleuropein and hydroxytyrosol on the host cells. Oleuropein and hydroxytyrosol exhibited no hemolytic activity on human erythrocytes at tested concentrations (3-60 μM). Antigenotoxic effects of oleuropein and hydroxytyrosol against H2O2-induced damage in human lymphocytes were evaluated in vitro by alkaline comet assay. Pretreatment of human lymphocytes with each of the substance for 120 min produced dose-dependent reduction of primary DNA damage in the tested cell type. At tested concentrations (1, 5, 10 μM), hydroxytyrosol showed better protective effect than oleuropein against H2O2-induced DNA breaks. Our results suggest that genoprotective effects of these phenolic compounds could be attributed to their radical scavenging and metal ions chelating activity which were determined using ABTS, FRAP and CUPRAC assays.