Aromatization of testosterone to estradiol was investigated in a human prostatic carcinoma cell line, LNCaP. A saturable, dose and time-dependent aromatization of testosterone was observed. Kinetic parameters, Km (201 nM) and V(max) (0.76 pmol/h) per mg) and also the inhibition constants (Ki) for various aromatase inhibitors were calculated from standard Lineweaver-Burke plots. The steroidal aromatase inhibitor 4-hydroxy-4-androstene-3,17-dione (4-OHA) and its derivative, 4-methoxy-4-androstene-3,17-dione (4-OMA), inhibited aromatization of testosterone in a competitive pattern of inhibition. The derivative 4-OMA is the stronger inhibitor of the two, with an apparent Ki of 1.12 microM, whereas the apparent Ki of 4-OHA is 3.28 microM. Long term incubation with 4-OMA suppressed proliferative activity of LNCaP cells in the presence of physiological levels of testosterone (10(-10) M to 10(-7) M). In contrast, 4-OHA was a growth promoter. These results suggest a potential role for aromatase in hormone responsive prostate cancer.