In this study, we used TCR isotype-specific antibodies to examine the frequency, phenotype, and histologic localization pattern of Tγδ cells in humans. The TCRδ1+ cells comprised an average of 15% of the splenic CD3+ cells and 7% of circulating T cells. The Tγδ cells in these human tissues, like their avian counterparts, were often not 'double-negative' for the CD4 and CD8 accessory molecules. Approximately 50% of the splenic δ+ cells expressed CD8, and 30% of the δ+ cells in blood were CD8+. T cells of both γδ and αβ TCR isotypes were exceedingly rare in the skin. The Tγδ cells exhibited preferential homing to the sinusoidal areas (red pulp) of the spleen and into the epithelial layer of the intestine in humans, as had been previously noted in chickens. Although 80% of the Tγδ cells in the human intestinal mucosa were localized in the epithelial layer, these cells represented only 5 to 10% of all the CD3+ T cells in this microenvironment. We conclude that Tγδ cells represent a sizeable subpopulation of the T cells in human peripheral tissues. The phylogenetic conservation of the CD8 expression by peripheral Tγδ cells and of their preferential homing pattern suggests a special role in bodily defense for this T cell subpopulation.