This investigation examined the effects of 6-methylene progesterone (6MP), an irreversible inhibitor of 5-alpha-reductase, on prostatic cancer (PC) cell lines. Dose titration microculture tetrazolium assays were used to evaluate cytotoxicity in cultures treated for 72 hr with 6MP (0-20 micrograms/ml). An androgen-sensitive cell line, LNCaP, was drug-sensitive with a mean 50% lethal dose value (LD50) of 2.632 +/- 0.103. Hormone-resistant PC cell lines 1-LN, DU 145, and PC3 also demonstrated sensitivity with LD50 values between 0.8579-1.110 micrograms/ml with a group average of 1.023 +/- 0.082 micrograms/ml. Increasing dosages of dihydrotestosterone in the growth media did not alter 6MP cytotoxicity in androgen-insensitive prostatic cancer cell lines. No correlation between androgen-responsiveness and 6MP-induced cytotoxicity was observed. In nonprostatic malignancies, 6MP inhibited adenocarcinoma cell lines with a mean group LD50 value of 0.7772 micrograms/ml +/- 0.110. J82, a transitional cell carcinoma cell line of bladder origin, exhibited an average LD50 value of 1.041 +/- 0.260. In an epidermoid cervical cancer cell line, ME180, an LD50 value of 0.5356 micrograms/ml +/- 0.010 was noted. In a melanoma cell line, Du Mel 6, a mean LD50 of 0.7428 +/- 0.023 micrograms/ml was achieved with 6MP. We conclude that 6MP, a novel 5-alpha-reductase inhibitor, has potential as a cytotoxic agent in prostatic carcinoma and additional human malignancies. Further study is justified.