Conjugated linoleic acid (CLA) has been shown to have a direct oncostatic action on MCF-7 human breast cancer cells in culture. However, the mechanism involved is not fully elucidated. In this study we have examined whether the inhibitor action is related to the estrogen responsiveness of MCF-7 cells. Our results demonstrate that CLA selectively inhibits proliferation of ER positive MCF-7 cells as compared with ER negative MDA-MB-231 cells. Cell cycle studies indicated that a higher percentage of CLA treated MCF-7 cells remained in the G0/G1 phase as compared to control and those treated with linoleic acid (LA). CLA also inhibited expression of c-myc in MCF-7 cells. These results demonstrate that CLA may inhibit MCF-7 cell growth by interfering with the hormone regulated mitogenic pathway. We are reporting for the first time the involvement of CLA, a dietary factor, in the regulation of hormone mediated mitogenic pathways in ER positive breast cancer cell proliferation in vitro.