Brown Cuban propolis (BCP) is the major type of propolis in Cuba; its chemical composition is exclusive and the principal component is nemorosone. In this study we investigated the antiproliferative activity of the ethanol extract of BCP on human breast cancer cell lines. The MTT assay showed a significant antiproliferative activity (P<0.005) of BCP on MCF-7 (estrogen receptor positive ER+) rather than MDA-MB 23 1 (ER-). This effect was concentration- (1-25 microg/mL) and time- (24-48 h) dependent, but it is only partially attributable to apoptosis. Indeed, our data showed that BCP administration to MCF-7 caused a significant (P>0.01) inhibition of cell growth in the G1 phase of cell cycle, which was mechanism dose- and time-dependent. 17-beta Estradiol (10 nM) administration to MCF-7 caused a significant (P<0.001), but not total reduction of BCP antiproliferative activity at concentrations of 1, 5 and 10 microg/mL, but not at the highest concentration (25 microg/mL). The coadministration of ICI 182,780 (100 nM), an antagonist of ER, on MCF-7 totally reduced the effect of BCP at 24 h, and showed a significant (P<0.001) reduction of BCP antiproliferative activity at 48 h. Thus it was hypothesized that BCP possesses an estrogen-like activity. It is to be noted, however, that BCP application to MDA-MB 23 1 at 48 h also induced increased cell mortality. Thus, it cannot be ruled out that BCP could not only interact with the ER, but also have an ER-independent activity.