We investigated the anticarcinogenic potential of green tea and its components epigallocatechin gallate (EGCG) and quercetin, as well as tamoxifen, on MCF-7 and MDA-MB-23 breast cancer cells. Using high-performance liquid chromatography, the quantity of EGCG and quercetin in green tea was analyzed. The receptor status of the cells was confirmed immunohistochemically. Various viability and cytotoxicity tests were later performed to investigate the effects of the substances. After incubating the cells with green tea extract, EGCG, quercetin and tamoxifen, a decrease in viability (MTT test) or proliferation (BrdU assay) was found in all cell tests with varying effects, depending on the assay used. The effects were similar in both cell lines. This work confirmed that EGCG and quercetin are contained in green tea and that both substances in pure form and as green tea have an anticarcinogenic effect on both estrogen receptor-positive and -negative breast cancer cells. This effect could also be demonstrated with tamoxifen in both cell lines (MTT and BrdU assays). These results suggest that the effects observed in these experiments are not generated only via estrogen receptor-mediated pathways.