The established cell lines from human prostatic cancer, such as Duke 145, 8PC93, and 19PC93, were examined in terms of their producing activity of acid phosphatase (ACP) and sensitivity to sex hormones. The results obtained are summarized. 1. ACP producing activity ACP was estimated with phenyl phosphate as a substrate. Values of the materials from each of the cells extracted with 5% Triton X-100 were Duke 145 (6.1 u/mg), 8PC93 (40.6 u/mg), and 19PC93 (40.4 u/mg), respectively. Activities of ACP were prohibited by the presence of L-tartrate. Histochemistry of ACP was demonstrated by azo-dye staining procedure, revealing the positive reactions in the cytoplasms of 8PC93 and 19PC93 cells, but weak reaction in duke 145 cells. Disk polyacrylamide gel electrophoresis (D-PAGE) was employed for ACP analysis of the cell extracts with 5% Tryton X-100 treatment. Two main bands were observed near original point and at another point proposed as ACP-2. These ACP positive reactions on the gels were also inhibited by the presence of L-tartrate in staining solution. In the case of Duke 145 cell material, the intensity of the reaction was observed weak in those specific two bands. 2. Hormone effects to the cells The prostatic cancer cells were examined in terms of sensitivity to sex steroid hormones such as androsterone, progesterone, estrone, estradiol, and estriol, by a colony formation method. Fifty percent reduction in colony formation of the 8PC93 and 19PC93 cells was found at the concentration of ca. 1.5 micrograms/ml in the case using progesterone or estrone, or estradiol, while 50% reduction of the Duke 145 cells was observed at 5 micrograms/ml only in a case using progesterone.