To investigate the effect of long chain n-3 polyunsaturated fatty acids on breast cancer cell growth, estrogen-dependent MCF-7 human breast cancer cells were cultured serum-free DMEM media containing 0.5$mu extrm{g}$/ml of differnet kinds of fatty acids; linoleic acid(LA), arachidonic acid(AA), eicosapentaenoic acid(EPA) and docosahexaenoic acid acid(DHA) and 1, 0.1, 0.2, 0.5and 1.0ng/ml 17$eta$-estradiol as well as 10$mu extrm{g}$/mi insulin and 1.25 mg/ml delipidized bovine serum albumin for 3 days. Cell growth monitored by MTT assay was lower in DHA and EPA treatments as compared to LA treatment, but not with AA treatment. Estrogen concentrations at which cell growth was initially stimulated were 0.1ng/ml for LA and DHA treatments and 0.2ng/ml for EPA and AA treatments, but the degree of stimulation was 25~30% lower in DHA and EPA treatments than in LA treatment. Fatty acid analysis showed that each fatty acid in culture medium was well incoporated into celluar lipid. Protein kinase C activity of cells was most elevated in LA treatment from 2 to 8 hours of culture followed by DHA, EPA, and AA treatments. It is concluded that inhibitions of n-3 DHA and EPA on breast cancer cell growth as compard with n-6 LA is mediated via changes in membrane fatty acid composition reducing estrogen sensitivity and increasing protein kinase C activity.