This study investigated the effect of anti-CD4 MoAb treatment on lymphocyte phenotype and function and correlated these changes with the prolongation of cardiac allograft survival in adult mice. Indefinite survival of heterotopic cardiac allografts was obtained in several fully allogeneic strain combinations when two doses of the anti-CD4 MoAb, YTS 191.1, were given at the time of transplantation. A dose response analysis in the C57BL/10 to C3H/He strain combination showed that very low doses of YTS 191.1 (25 micrograms/dose) were able to induce prolonged allograft survival when administered perioperatively. At the time of transplantation the immunosuppression induced by administration of the anti-CD4 MoAb is not antigen-specific, as heart grafts from different donor strains, mismatched for both major and minor histocompatibility antigens, showed prolonged survival in treated recipients. Immunocompetence was restored by 6 weeks after MoAb treatment, as recipients regained the ability to reject a cardiac allograft transplanted at this time point. However, while recovery of immunocompetence could be demonstrated in vivo, leucocytes isolated from the peripheral lymphoid organs of treated mice continued to be hyporesponsive in mixed leucocyte culture (MLC). Phenotypic analysis of the peripheral lymphoid tissues showed that C3H/He recipients treated with 25 micrograms/dose of YTS 191.1 had a marked, but not complete, elimination of the CD4+ subset at the time of transplantation, which was gradually restored to 50% of normal by 6 weeks after treatment. Thus, complete elimination of the CD4+ subset was not required to achieve indefinite allograft survival, and immunocompetence, as assessed in vivo, returned even when the CD4+ subset was present at half the normal level. Low doses of anti-CD4 MoAb (25 micrograms) had no effect on the expression of the CD4 molecule by thymocytes, and yet thymocytes were hyporesponsive to alloantigen in vitro. At higher doses of YTS 191.1, immature CD4+8+ thymocytes were selectively depleted. These results suggest that anti-CD4 MoAb therapy may modulate the intrathymic T cell selection process. These studies provide further insight into the mechanism of action of low dose, depleting anti-CD4 MoAb therapy in allograft rejection, and form a basis from which rational modifications to therapeutic protocols in transplantation models can be made.