Aim of the study: The aim of this research is to examine the potential cytotoxic and antitumor effect of the essential oil isolated from the leaves of the Pistacia lentiscus L. plant on two human breast cancer cell lines - MDA-MB-231 and MCF-7. It was hypothesized that the number of cancer cells will decrease after exposure to certain concentrations of essential oil of mastic tree compared to the control group. Materials and methods: In order to measure the percentage of surviving cells, i.e. cellular metabolic activity, a non-radioactive colorimetric MTT test system was used, which is based on the reduction of yellow tetrazolium salt into purple formazan crystals by metabolically active cells. Essential oils were incubated in the concentration range from 1 to 500 µg/mL for 72 hours in three replicates, and the absorbance was measured at 570 nm on a microplate reader after 24, 48 and 72 hours. Results: The results of this research after exposure of cancer cells to different concentrations of the essential oil of the mastic tree differ for each breast cancer cell line. The most significant results of the effect on the MDA-MB-231 cell line are after incubation for 48 hours at a concentration of 500 µg/ml where cell viability decreased to 19% and at a concentration of 250 µg/ml after 24 hours where cell viability decreased to 52 %. In the case of the MCF-7 cell line, the most significant results were obtained after incubation for 48 hours at a concentration of 500 µg/ml, where cell viability decreased to 30%, and at a concentration of 500 µg/ml after 24 hours, where cell viability decreased to 36%. Conclusion: Based on the results of this research, we conclude that the essential oil of the Pistacia Lentiscus L. leaf has potential in the treatment of breast cancer and that it kills cancer cells in a certain percentage. Compared to the MDA-MB-231 cell line, mastic tree leaf extracts show a weaker cytotoxic effect on the MCF-7 cell line. The IC50 value was reached for both cell lines and it decreases with increasing incubation time. The value is lower in MDA-MB-231 cell line compared to MCF-7 cell line.