The immunopathologic findings in the testes and kidneys of 25 long-term vasectomized monkeys and of 13 age-matched control animals were compared. Antisperm antibody was found in serum samples from 60% of vasectomized monkeys and in no samples from control animals. Orchitis, aspermatogenesis, or both, resembling allergic orchitis, was noted in 92% of vasectomized animals and in 23% of controls. Epididymitis and epididymal granuloma occurred exclusively in the vasectomized animals (52%). By immunofluorescence, granular deposits of IgA, IgG, and/or C3 in the basal lamina of the ductuli efferentes and the caput epididymidis were detected in 45% of the control monkeys. In vasectomized animals, the incidence of immune deposits increased to 91%, and the deposits became more widespread. Although both vasectomized and control animals had IgM, IgG, or both, in the glomerular mesangial region, C3 was found in vasectomized monkeys only (29%). The results of this study indicate that monocytic infiltrations with aspermatogenesis in association with immune deposits along the excurrent duct normally occur in subhuman primates, and that these changes increase in incidence after vasectomy. Although the testicular disease highly resembles allergic orchitis, an immunologic basis remains to be established.