Colony-forming epithelial cells can be separated from the non-dividing “foam cells” in human milk by differential adhesion to glass and freezing. The growth of such partially purified mammary epithelial cells is stimulated by co-culture with non-dividing feeder cells. Foam cells, mitomycin-treated mouse fibroblast lines and human mammary fibroblasts and calf lens epithelial cells are all effective in promoting mammary epithelial cell growth. Contact between epithelial cells and feeders is not required for the growth-promoting effect. The mitogenic effect of epidermal growth factor on mammary epithelial cells also requires feeder cell activity.