Ten to 23% of cells in blood, lymph node and bone marrow from normal dogs formed rosettes with human erythrocytes, and 12-27% formed rosettes with erythrocyte-antibody-complement (EAC) complexes. In contrast, only 3% of thymocytes, and 1% of thoracic duct cells formed rosettes with human erythroyctes, and 0 and 15% respectively formed EAC rosettes. When peripheral blood mononuclear cells were separated by rosette sedimentation into populations depleted of, or enriched for, cells forming rosettes with human erythrocytes (H-RFC), the population depleted of H-RFC responded more vigorously to alloantigens in mixed leukocyte culture (MLC) (P < 0.01) and to the mitogens phytohemagglutinin (PHA) (P = 0.01) and concanavalin A (P = 0.01) than did the population enriched for H-RFC. Passage of peripheral blood mononuclear cells over nylon wool columns produced a nonadherent population depleted of H-RFC, EAC rosette-forming cells and cells binding surface immunoglobulin (SIg), while the adherent population was enriched for each of these markers. In 3 dogs 36%, 44% and 64% of adherent cells that formed rosettes with human erythrocytes also possessed SIg, suggesting that canine B cells form rosettes with human red cells. The nonadherent population showed a more vigorous response to alloantigens in MLC (P < 0.01) and to PHA (P < 0.05) than the adherent population, and also stimulated the growth of autologous erythroid colonies better than the adherent population (P = 0.02). A T cell rich population can thus be obtained from canine peripheral blood, but no specific marker for T cells has been identified. Specifically, the capacity to form rosettes with human red cells is not a marker for the canine T cell.