Recently developed culture systems that allow extended growth of normal rat mammary epithelial (RME) cells were used to directly compare the proliferative potentials and growth factor requirements of primary normal and primary neoplastic rat mammary cells. RME cells were obtained from 45- to 55-day-old inbred female Lew rats and rat mammary tumor (RMT) cells from 7,12-dimethylbenz(a)anthracene- or N-nitroso-methylurea-induced mammary carcinomas. To compare the proliferative lifespan of RME and RMT cells, colony forming efficiencies were determined after consecutive passages over a 70-day culture period. Whereas the proliferative potential of RME cells declined with time in culture, RMT cells from five separate mammary carcinomas had colony forming efficiencies that increased with serial passage. By the end of the 70-day culture period, colony forming efficiencies for RMT cells were 100- to 1000-fold higher than those for RME cells. To compare the growth factor requirements for RME and RMT cells, a serum-free culture medium that supports RME cell growth was used and the influence of specific growth factors was examined by deletion experiments. Cells from 5 of 18 primary mammary carcinomas exhibited requirements for insulin, epidermal growth factor, and cholera toxin identical to those of RME cells. In contrast, cells from 9 of 18 tumors expressed independence of one, two, or all three of these factors for growth in serum-free culture. To examine the in vivo growth potential of primary RMT cells, samples of the cell suspensions used to initiate primary cultures were transplanted into the interscapular white fat pads of syngeneic female recipients. Transplantation of cells from growth factor dependent tumors yielded nonneoplastic mammary outgrowths. In contrast, transplantation of growth factor independent tumor cells yielded grossly visible tumors in 100% of the recipients within 4 weeks of transplantation. Thus, our results indicate that cells from all primary mammary carcinomas have dramatically enhanced growth potential in long-term culture relative to RME cells. Furthermore, a subset of these tumors are also independent of growth factors required by RME cells, and expression of growth factor independence is associated with high neoplastic potential in vivo.