Mifepristone (RU 486), used clinically for the termination of early pregnancy, and its acetyl and 13-retro (13 alpha) analogs show potent antiproliferative effects against estrogen-dependent human breast tumors and endometriosis. However, there has been no report on direct inhibition of aromatase by antiprogesterones. Aromatase inhibitors have been shown to be effective against estrogen-dependent breast cancer. We evaluated the inhibition of aromatase by various antiprogestins (ZK 112.993, ZK 98.734, ZK 114.043, ZK 98.299, and ZK 114.863). Human placental microsomes were incubated with [1 beta-3H,4-14C] androstenedione (3-114 nM) in the presence of NADPH, with or without putative inhibitors (10-200 microM). Aromatase activity was assessed by tritium release to water from the 1 beta-position of the substrate. ZK 112.993 and ZK 98.734 did not show any inhibitory effect. The statistical analysis of the data using standard errors was obtained from replicate experiments. ZK 114.043 showed slight inhibition with a Ki of 54.8 +/- 6.4 microM (m +/- SE, n = 6) against androstenedione aromatization. The two 13-retro-steroids, ZK 98.299 and ZK 114.863, showed aromatase inhibition with Ki values of 19.0 +/- 1.5 microM (n = 7) and 12.7 +/- 0.94 microM (n = 7), respectively, which is weak with respect to some known potent inhibitors, but significant when compared with the other antiprogestins which were tested. The results suggest that the unnatural 13-retro-antiprogestin conformation may have a better fit to the aromatase active site than the natural 13 beta-antiprogestin conformation. (Steroids 60:234-238, 1995).