Differential involvement of CD4+ cells in mediating class I-disparate skin graft rejection was investigated using quantitatively different Kb transgenic mice as donors under conditions in which CD8+ cells were blocked in vivo by administration of anti-CD8 monoclonal antibody (mAb). Tg.H-2Kb-1 and -2 are C3H transgenic mice with 14 and 4 copies, respectively, of the H-2Kb gene. Cell surface expression of Kb antigen and the Kb antigenicity of skin for eliciting graft rejection with homozygous and heterozygous transgenic mice were correlated with the copy number. In vivo administration of anti-Lyt-2.1 (CD8) mAb markedly prolonged survival of heterozygous and homozygous C3H Tg.H-2Kb-2 skin grafted onto C3H mice, but prolonged survival of heterozygous Tg.H-2Kb-1 skin grafts much less and did not prolong survival of homozygous Tg.H-2Kb-1 grafts. Administration of anti-L3T4 (CD4) mAb alone did not have any effect on skin graft rejection. Administration of anti-L3T4 (CD4) mAb with anti-Lyt-2.1 (CD8) mAb blocked rejection in all combinations. These findings indicate that a quantitative difference of class I antigen caused differential activation of CD4+ cells under conditions in which CD8+ cells were blocked.