We investigated the influence of fetal bovine serum, complemented or otherwise with estradiol or insulin or both, on the proliferation of mammary cancer cells from long-term and primary cultures. The long-term culture corresponded to mouse MXT and MCF-7 cell lines whereas the primary culture corresponded to primitive breast cancers squashed onto histologic slides and maintained in cultures for between 12 and 48 h. Cell proliferation was evaluated by means of digital cell image analysis of Feulgen-stained nuclei. Our results show that the addition of estradiol and insulin slightly but nevertheless significantly increases the proportion of cells still living at Hour 48 of culture. Fetal bovine serum, necessary for the growth of MXT and MCF-7 mammary cells, was highly cytotoxic with respect to the primary cultures of the 20 breast cancers under study. We are now conducting new experiments using chemically defined media to study the influence of new antineoplastic compounds on primary cultures of breast cancers.