Allograft unresponsiveness can be induced by donor bone marrow cells (BMC) in antilymphocyte serum (ALS)-treated recipients. The effect of administering monoclonal anti-CD4 and -CD8 at several points has been examined in a mouse skin allograft model of this protocol. Brief peritransplant administration of anti-CD4 and -CD8 was used to replace ALS. Anti-CD4 treatment prolonged graft survival only slightly and conditioned recipients poorly for the effect of posttransplantation donor BMC infusion. Anti-CD8 was ineffective in both capacities. A mixture of anti-CD4 and anti-CD8 was at least as effective as ALS in prolonging graft survival and in promoting the beneficial effects of donor BMC. Like the monoclonal antibodies, ALS also depleted splenic and lymph node CD4+ and CD8+ cells. Injection of ALS, but not the monoclonal antibodies, altered the CD4/CD8 phenotype of thymocytes, although persistent binding of both types of antibody to thymocytes was demonstrated. Abrogation of the positive effect of BMC by reconstitution of normal spleen cells on day +3 after ALS treatment confirmed that cell depletion is a requirement of this system. Monoclonal antibodies were also given to ALS/BMC-treated recipients after their grafts had become established. Anti-CD8 injection at either 2 or 4 weeks after transplantation further prolonged graft survival. In contrast, anti-CD4 injection at 2 or 6 weeks after grafting precipitated rejection, which suggests that continued allograft survival following ALS and donor BMC treatment is due to the activity of a CD4+ cell population.