The cytostatic effect of medroxyprogesterone acetate (MPA; an oral luteohormone preparation) on two cell lines of ovarian carcinoma cultured in vitro (SHIN-3 and MN-1) was studied. At the same time, changes in the morphology and colony formation of these cancer cells after exposure to the drug were examined. 1) The doubling time of SHIN-3 cells in the logarithmic growth phase was prolonged 2.5 times at a MPA dose of 10(-8)M and 3.2 times at a MPA dose of 10(-5)M. The drug, however, exerted no significant cytostatic effect on MN-1 cells. 2) When the IC50 of MPA was assessed by counting live SHIN-3 cells in an FCS-added medium, it was 2.7 x 10(-5)M. When the same assessment was done with a medium without serum, IC50 was 6.4 x 10(-6)M. 3) After 120 hours of incubation in a medium containing 10(-8)M of MPA, CA125 (a tumor marker) production by SHIN-3 cells was suppressed by 35%. The suppression rate was 79% for SHIN-3 cells incubated in a medium containing 10(-5) of MPA. 4) The cytoplasm of SHIN-3 cells, incubated in a MPA-added medium, showed the formation of small mucous vacuoles and expansive degeneration, accompanied by a marked increase in size and thinning of the nucleus. 5) In an experiment on colony formation with collagen gel, the colony size decreased MPA concentration dependently, and was accompanied by the appearance of lobulated colonies.(ABSTRACT TRUNCATED AT 250 WORDS)