Steroid hormones and peptide growth factors promote growth and development of normal mammary tissues and some types of breast cancer. Ovarian steroids may influence mammary growth directly or indirectly. The epidermal growth factor (EGF) family of proteins may also regulate mammary growth. These two pathways may function independently of each other or they may act in concert, with steroids inducing transcription of genes that encode growth factors or growth factor receptors. We used a feline mammary adenocarcinoma cell line (K12) to address whether there was an interrelation between progesterone (PGN) and EGF-associated growth pathways. K12 cells responded to EGF by a dose-dependent increase in proliferation. PGN or promegestone (R5020, a synthetic progestagen) alone did not stimulate K12 growth, but when EGF and PGN, or EGF and R5020 were combined, they were synergistic. This synergistic response was abrogated by the PGN receptor antagonist RU486 or by antibodies that blocked binding of EGF to its receptor. K12 cells expressed characteristic double-affinity EGF receptors, as well as p l 8 5 (a functionally and structurally related protein, product of the neu gene) on their surface. PGN receptors were also found on intact cells and in cleared cytosols. Stimulation of K12 cells by PGN or by R5020 induced a two-to threefold increase in the number of high-affinity surface EGF receptors after 24 h. Stimulation of these cells by PGN also affected the relative levels of phosphorylation of the EGF receptor and p l 8 5 within minutes, but not of other cellular phosphoproteins. Our results show that PGN enhances the EGF-induced growth of K12 cells and suggest that this effect may be mediated at least partly via an increase in the number or function of high-affinity EGF receptors.