Leukocytes, specifically macrophages, lymphocytes and mast cells, are found within the testes of most, if not all, mammals. In some species (for example, rats, mice and humans), the number of 'resident' testicular macrophages, in particular, is quite considerable. However, reproductive biologists are only beginning to explore the characteristics and possible biological significance of these cells. As in other tissues, the testicular leukocytes are involved in immunological surveillance, immunoregulation and tissue remodelling. They are implicated in the mechanisms that make the testis a particularly successful site for tissue transplantation in some experimental animals. Moreover, recent studies have demonstrated that the testicular macrophages have specific trophic effects on Leydig cell development and steroidogenesis. In turn, the development and functions of the testicular leukocyte population are clearly influenced by the testicular environment, and especially by the Leydig cells and Sertoli cells. These data indicate an important role for leukocytes in testicular homeostasis. Balanced against this beneficial role is the fact that these cells possess the potential to damage testicular function in conditions of immune activation, as their inflammatory and cytotoxic activities may disrupt the normal environment of the testis. The importance of the testicular leukocytes to normal and abnormal testicular function is evident. The challenge for future research is to define the details of this relationship.