As 5-fluorouracil (5-FU) has been known to show clinically antitumor effects against both breast and stomach carcinomas. We compared the combined effects of medroxyprogesterone acetate (MPA) with 5-FU on the growth of cultured human breast and stomach carcinoma cells. MPA inhibited the growth of estrogen-dependent human breast carcinoma MCF-7 cells at the low concentrations and exhibited an additive effect in combination with 5-FU. MPA also inhibited the growth of human stomach carcinoma MKN-45 cells at relatively high concentrations and exhibited an additive effect in combination with 5-FU. Human stomach carcinoma MKN-28 cells were rather insensitive to MPA, but, an additive combination effect of MPA and 5-FU was observed. These three cell lines were found to have MPA-binding proteins which may be distinct from the nuclear progesterone receptor, suggesting the correlation with growth-inhibitory activity of MPA.