Pregnancy-associated plasma protein-A (PAPP-A) has been found in seminal fluid at concentrations 10-20 times higher than in plasma. This observation prompted us to undertake a morphological study of the male genital tract with the aid of the immunoperoxidase technique. Immunoreactive cells were found among the Leydig cell population, as well as among the epithelium of the rete testis, the head of the epididymis and the seminal vesicles. The relatively small number of weakly staining epithelial cells leaves open the question of whether PAPP-A is excreted by these cells or whether an active transfer mechanism-possibly in the seminal vesicles-is at work, the Leydig cells increasing the plasma concentration locally, by secretion of PAPP-A into the circulatio
