Steroid concentrations were measured in follicular fluid obtained from patients fertilized in vitro. Progesterone and estradiol-17β concentrations showed positive correlations with the growth of follicles, whereas the testosterone concentration had negative correlations not only with follicle growth but also with progesterone and estradiol-17β concentrations. The testosterone concentration was significantly lower in follicles with cleaved oocytes than in those with uncleaved oocytes or those with cleaved but degenerated oocytes. After the luteinizing hormone (LH) surge, the fluid contained significantly higher progesterone and lower testosterone concentrations than after human chorionic gonadotropin (hCG) injection. These results indicate that steroid concentrations vary with the growth of follicles, and that the oocyte that can cleave well tends to be associated with a low testosterone concentration. Furthermore, the LH surge seems to provide a better hormonal condition for cleavage than does hCG injection.