Effects of epostane on human trophoblast cells, co-culture of mouse embryos with endometrium and guinea pig corpora lutea were studied in vitro. Human chorio-trophoblast cells were obtained from induced abortion (2-2.5 months of pregnancy). The tissue was incubated in control medium or in medium containing epostane. The results indicated that epostane at a concentration of 50 micrograms/ml could injure the human trophoblast cells and its function. Obviously damaged cells were observed at the concentration of 100 micrograms/ml. The cells were degenerated with pyknotic nuclei and sometimes only cell fragments were found. Guinea pig corpora lutea produced progesterone in vitro. In the incubation medium of the control group, the total luteal progesterone increased linearly with the prolongation of incubation during the first 30 min of incubation. The addition of hCG at concentration of 10 IU/ml clearly accelerated progesterone production. Epostane at a concentration of 50 micrograms/ml could significantly inhibit both basic and the hCG-stimulated luteal production of progesterone. Epostane at concentrations of 50 and 100 micrograms/ml showed no inhibitory effect on mouse embryos and endometrium in vitro.