Objective: The aim of the research was to examine the potential cytotoxic effects of phenolic compounds isolated from Leccino and Frantoio olive buds on two cell lines of human breast cancer: MDA-MB-231 and MCF-7. The number of carcinoma cells was assumed to be reduced after exposure to phenolic compounds in comparison with the control group. Materials and methods: Cytotoxic effects of phenolic compounds on breast cancer cell lines were determinged with MTT assay, after the periods of 4, 24, 48 and 72 hours of incubation. The comparison between the absorbance of created formazan in cells treated with samples obtained from the olive buds and the absorbance of the control group (cells untreated with olive bud samples) established the survival rates of carcinoma cells. Results: The results of breast cancer cells (cell lines MDA-MB-231 and MCF-7) exposure to olive extracts E3 (Leccino sort) and E4 (Frantoio sort) are shown graphically as relation of the percentage of metabolically active cells to the concentration of the compound. The most significant effect on the MDA-MB-231 cell line was achieved after incubation of 48 hours at a concentration of 200 μg/mL for both sorts, and on the MCF-7 cell line after incubation of 72 hours at a concentration of 100 μg/mL for the E3 sort and concentration of 200 μg/mL for the E4 sort. Conclusion: Olive bud extracts show a certain cytotoxic effect on human breast cancer cell lines MDA-MB-231 and MCF-7, which depends on exposure time and concentration, and to a certain extent reduce the survival of said cancer cells. The effect of extracts is not always proportional to the increase in concentration. The extracts of the cultivars used in our research did not reach the IC50 value.