Multicellular tumor spheroids, an in vitro 3-D model that simulates malignant-cell contacts within a tumor, can be used to evaluate tumor response to therapeutic agents. We found that MELN (derived from MCF-7 cells) cells grown in 3-D as spheroids, remain highly sensitive to estradiol in terms of growth, down-regulation of ERalpha expression and ERalpha-induced transcriptional activity. Estradiol induces cyclin D1 and CDK1 proteins in Ki-67 positive proliferating cells, whereas survivin is up-regulated in both Ki-67 positive proliferative outer layer of cells and around the necrotic zone in non-proliferating cells. OH-Tam inhibits both estradiol-induced transcriptional activity and estradiol-dependent growth of MELN spheroids. Consistent with its antiproliferative effect, we observed that OH-Tam induces an important decrease in the proportion of proliferating cells, positive for Ki-67, cyclin D1 and CDK1. But, in contrast to what was expected, OH-Tam treatment resulted in a decrease in the proportion of p21 positive cells. Furthermore, despite its ability to down-regulate survivin in MELN spheroids, OH-Tam did not trigger apoptosis. Taken together, these results indicate that this model, is more relevant to an in vivo situation than monolayer cultures. It could be useful to identify new markers of the response to endocrine treatment and to investigate the effects of drugs combination.