The crucial role of the immune system in the defence against infection and malignancy and its intimate involvement in the pathogenesis of a wide variety of chronic diseases is well established. Although the male genital tract is particularly vulnerable to infection, malignancies and autoimmune disorders which may lead to infertility, our knowledge of the distribution of cells of the immune system in the genital tract is limited. We have therefore investigated this matter using monoclonal antibodies to lymphocyte subsets, macrophages and HLA-DR antigens. These studies have revealed a consistent pattern of distribution of the cells of the immune system within the tissues of the male genital tract. Of special interest were the predominance of the T-suppressor/cytotoxic cells in the lining epithelium of the genital tract, the demonstration of macrophages in testicular tissues and the preferential expression of HLA-DR antigens on the lining epithelium. Profound changes in this pattern were noted in clinical specimens examined. These observations throw light both on normal immune homeostatic mechanisms within the male genital tract and the pathogenicity and aetiology of certain genital tract disorders and highlight the potential value of further studies in this area.