The proliferative responses of human prostatic carcinoma have been evaluated in organ culture using [125I]-iododeoxyuridine ([125I]UdR) to monitor DNA synthesis. The morphological preservation was not influenced by the addition of fetal calf serum or insulin (5 mu gm/ml), transferrin (10 mu gm/ml), and thyrotropin releasing hormone (10(-9) M) to the active medium. Testosterone (4 X 10(-7) M) stimulated [125I]UdR uptake, whereas diethylstilboestrol (4 X 10(-6) M) had no direct effect on uptake. Both estramustine phosphate (4 X 10(-6) M) and oestradiol-17 beta (4 X 10(-6) M) inhibited uptake in a similar manner. Thus while explants of human prostatic carcinoma derived from transurethrally resected specimens can be well maintained in organ culture for a few days, proliferative responses are small and difficult to measure for individual patients.