The effect of estramustine (EM), estradiol-17 beta (E2) or 5 alpha-dihydrotestosterone (DHT) on the growth of two human prostatic carcinoma cell lines, LNCaP and LNCaP-r was investigated. The hormone resistant subline LNCaP-r was derived in our laboratory, from the hormone sensitive LNCaP cell line. E2, 10(-8) or 10(-5) M inhibited the growth of the LNCaP cells, but did not affect the LNCaP-r. DHT, 10(-8) M, had a stabilizing effect at the stationary phase on the growth of the LNCaP cells whereas at higher concentrations, 10(-5) M, the growth rate was decreased. The LNCaP-r cell line was previously reported to be unaffected by DHT. EM inhibited the growth of both cell lines but LNCaP was more sensitive than LNCaP-r. E2 and DHT modulated the effect of EM. When treated with 10(-7) M EM, addition of E2 or DHT (10(-7)-10(-5) M) further inhibited the growth. When EM was used at a higher concentration (10(-5) M), the enhanced effect of growth inhibition by hormone addition was lost. Based on these results it is suggested that the presence of endogenous hormones, or estrogens released from EM on hydrolysis, may play a contributory role in the cytotoxicity of estramustine.