Macrophages are found throughout the male reproductive tract and its accessory glands. Mice homozygous for a null mutation (csfm(op)) in the gene for the mononuclear phagocytic growth factor colony-stimulating factor-1 (CSF-1) have a significantly lower density of macrophages, defined by the mononuclear phagocytic antigen F4/80, in the testis, cauda and caput epididymis, prostate, seminal vesicles, and vas deferens. These data indicate that CSF-1 is the major growth factor regulating the occurrence of macrophages in male reproductive tissues. The residual macrophages were correctly located in the tissue except in the caput epididymis, where they failed to take up positions adjacent to the tubular epithelium. Restoration of circulating CSF-1 concentrations in csfm(op)/csfm(op) males totally restored F4/80+ cell density in the testis and caput and cauda epididymis and partially restored their density in the vas deferens and seminal vesicles but failed to affect density in the prostate. This failure to correct all populations with circulating CSF-1 suggests the requirement for local synthesis of CSF-1 at appropriate developmental stages and/or its expression in a cell surface-associated form. The absence of macrophages in the testis and epididymis of csfm(op)/csfm(op) mice correlates with dysfunction in these tissues, suggesting that macrophages play important nonimmunological roles in these tissues.