A series of monoclonal antibodies that react with human lymphocyte subsets was used in an indirect immunoperoxidase technique to study representative blocks from normal human testis, epididymis, vas deferens, prostate and seminal vesicles. Biopsies of testis, epididymis and vas obtained during surgical procedures directed at the investigation and treatment of infertile males were also studied. In all normal tissues, apart from the peripheral testis where no lymphocytes were identified, T lymphocytes were the predominant cell type (Leu 4+). These lymphocytes were largely of the suppressor/cytotoxic phenotype (Leu 2a+) and were more abundant in between the epithelial cells in the rete testis, epididymis, vas deferens, seminal vesicles and prostatic acini. Cells of the helper/inducer phenotype (Leu 3a+) were identified mainly within the interstitium of the epididymis and the prostate. B-lymphocytes (Leu 12+) were few in number and were mainly in the stroma of the prostate. In each organ the ratio of the T-cell subsets was determined and changes in this ratio were observed in epididymal and vasal biopsies from some infertile males. Finally, in testis biopsies from infertile men, suppressor/cytotoxic T-cells were demonstrated between the germinal epithelium and the fibrous tunica of the seminiferous tubules and as focal aggregates in the interstitium.