The presence of thymidine phosphorylase in human healthy, adenomatous and cancerous prostate was demonstrated. The enzyme was responsible for the cleavage and synthesis of thymidine and for the transfer of deoxyribose from one deoxyribonucleoside to a pyrimidic base. The enzyme from normal and adenomatous prostate was retained on DEAE-Sephadex gel. In PC-3 cells, two enzymes with thymidine phosphorylase activity were present, one was retained on the gel, the second was excluded from it. Thymidine phosphorylase activity was higher in adenomatous and cancerous tissues that in healthy ones. In all tissues, the reactions of thymidine synthesis and of deoxyribose transfer were more important than that of thymidine cleavage.