Antigens of human (HIV) or simian immunodeficiency viruses (SIV) were identified with polyclonal or monoclonal antibodies and avidin-biotin complex (ABC) immunohistochemistry in fixed surgical pathology and autopsy specimens of humans or monkeys with the acquired immunodeficiency syndrome. With B-5 fixative, viral antigens were readily detected in lymph nodes of 8 of 13 patients with follicular hyperplasia, but in only 1 of 12 patients with follicular atrophy. Antigen was detected in follicular dendritic reticular cells and rare blastlike cells, extracellularly, and in postcapillary venules, medullary lymphocytes, sinus histiocytes, and macrophages in some lymph nodes. In the brain at autopsy, antigen could be found in gliomesenchymal-cell nodules, astrocytes, vascular endothelial cells, multinucleated cells, and astrocytes and macrophages associated with demyelination. In contrast, 4 rhesus monkeys with experimental SIV infection had abundant antigen in sinus histiocytes, macrophages, and multinucleated giant cells of lymph nodes and spleen and in thymic epithelial cells. Brain lesions of monkeys resembled those of humans, with antigen found in macrophages and multinucleated giant cells. Antibodies to HIV also were immunoreactive in formalin-fixed tissue sections of monkeys containing SIV antigens. The ABC technique provided a fast and efficient method for localizing HIV and SIV antigens in fixed surgical and autopsy specimens. These findings are consistent with those found with in situ hybridization, ultrastructural studies, frozen sections of lymph nodes, and permanent sections of brain.