The search for effective plant-derived anti-cancer agents or their synthetic analogs has continued to gain interest in drug development. The anti-cancer activity of parthenolide (PTL) isolated from Tanacetum parthenium, has been attributed to the presence of α-methylene-γ-lactone skeleton. In the present study we aimed to investigate the anti-cancer potential of a new synthetic compound, 3-isopropyl-2-methyl-4-methyleneisoxazolidin-5-one (MZ-6), with the same as in PTL α-methylene-γ-lactone motif, on two breast cancer cell lines, MCF-7 and MDA-MB-231. For comparison, PTL was included in the study. PTL and MZ-6 reduced the number of viable MCF-7 and MDA-MB-231 cells, with half maximal inhibitory concentration values between 6 and 9 μM. Both compounds dose-dependently inhibited incorporation of [(3)H]thymidine, up-regulated Bax and down regulated Bcl-2 mRNA. The levels of the end product of lipid peroxidation, malondialdehyde, were significantly higher. In MCF-7 cells, MZ-6 induced early apoptosis and cell cycle arrest in G0/G1 phase. The effect produced by MZ-6 was much stronger compared with PTL. In MDA-MB-231 cells, both tested compounds had similar effect and induced mostly late apoptosis. In conclusion, the observed anticancer activity makes MZ-6 an attractive drug candidate and shows that simple analogs of α-methylene-γ-lactones can be good substitutes for more complex structures isolated from plants.