We have investigated the relative roles of the Lyt-2+ and L3T4+ T lymphocyte subsets in rejection of class-I-MHC-antigen-disparate skin and tumor allografts. To deplete T cells in vivo, rat anti-Lyt-2 or anti-L3T4 monoclonal antibodies (mAb) were administered to adult-thymectomized (ATX) recipient mice prior to transplantation. BALB/c (H-2d) recipient mice rejected the Ia- Sarcoma I (Sa1) (H-2a) tissue culture-derived tumor after depletion of the L3T4+ T cell subset in vivo. In contrast, depletion of the Lyt-2+ T cell subset permitted lethal tumor growth in all recipient mice. To determine the role of particular T cell subsets in rejection of Ld class-I-MHC-antigen-disparate allografts, BALB/c skin was transplanted to BALB/c-H-2dm2 recipient mice. Skin grafts were rejected by control mice with a mean survival time (MST) of 14.5 days. The MST of skin grafts for mice treated with anti-L3T4 mAb was 16.6 days. In contrast, administration of anti-Lyt-2 mAb alone (MST = greater than 47 days) or together with anti-L3T4 mAb (MST = greater than 50 days) caused prolonged or indefinite graft survival in all recipient mice. Depletion of specific T cell subsets was confirmed by flow cytometric analysis and by analysis of T cell function in vitro. These results suggest that Lyt-2+ T lymphocytes are essential for rejection of class-I-MHC-disparate allografts; indirect presentation of alloantigen to L3T4+ T cells may not be necessary for rejection.