In the MCF7 human breast cancer cells, estrogens stimulate in vitro cell proliferation and biosynthesis of specific proteins which are released into the culture medium; the most abundant of these is a 52,000 dalton glycoprotein. Conditioned media from E2[estradiol]-treated MCF7 cells stimulate the growth of E2-derived MCF7 cells, whereas conditioned media from untreated control cells are ineffective. At protein concentration > 5 .mu.g/ml, an inhibitory effect was also obtained with control media. The mitogenic factors of these conditioned media are non-dialyzable, sensitive to temperature and proteases and adsorbed on Concanavalin A Sepharose. The cell surface of MCF7 cells, examined by scanning electron microscopy, is differentially affected by these 2 types of conditioned media. Estrogens may stimulate the growth of breast cancer cells indirectly via the induction of glycoproteins acting as growth factors.