In the present study the combined effect of in vitro tamoxifen and Avemar treatment was studied on MCF-7 (ER+) breast cells as a model of a breast cancer situation. Cells were transformed for 24, 48 and 72 hours, cytotoxicity was measured by MTT assay, the percentage of apoptosis and cell proliferation was determined by flow cytometry, hematoxilin/cosin staining and by immunochemistry using the ApopTag reaction. Estrogen receptor activation was studied by semi-quantitative determination of the estrogen-responsive pS2 gene mRNA production. Tamoxifen had no effect on apoptosis but significantly reduced the percentage of the S-phase. Avemar significantly increased apoptosis after 48 hours of the treatment. Apoptosis was increased when Avemar was added to tamoxifen treatment, but tamoxifen induced reduction in cell proliferation remained decreased. Estrogen receptor activity of MCF-7 cells was enhanced by Avemar treatments. Results suggest that the use of the supplementary therapy with Avemar in the case of ER+ breast cancer tumors is not contra-indicated and it may enhance the effect of tamoxifen on ER+ breast tumors.