Using a model system in which mature lymphocytes were adoptively transferred into immunodeficient C.B17-scid/scid recipients, the requirement for CD4+ T cells in rejection of previously healed-in multiple minor-H-antigen-disparate skin grafts was investigated. Depletion of functional CD4+ cells was accomplished by anti-CD4 antibody + complement treatment prior to adoptive transfer followed by chronic anti-CD4 serotherapy in vivo. Homozygous scid mice harboring nondepleted naive donor cells effectively rejected minor-H-antigen-disparate grafts, whereas scid/scid mice harboring CD4+ T-cell-depleted naive cells did not. Homozygous scid mice harboring minor-H-antigen-primed cells rejected the minor-H-antigen-disparate allografts regardless of whether or not the animals had received anti-CD4 treatment, although rejection by the mice receiving anti-CD4 treatment was retarded compared to mice not receiving anti-CD4 treatment. All the mice that received viable donor cells rejected minor + H-2 disparate allografts, demonstrating effective immunity in this case was not dependent upon the activity of CD4+ T cells. These data suggest that in responses against multiple minor-H-antigen-disparate tissue, primary allograft rejection is absolutely dependent upon CD4+ cells, and secondary allograft rejection is not. The implications of these findings in understanding interactions of T cell subsets in vivo and of the utility of using homozygous scid mice as recipients for exploring the function of transferred lymphoid cell populations are discussed.