In the present study, we have investigated the molecular basis for the immunoregulatory function of CD8 cells after autologous mixed lymphocyte reaction (AMLR) activation. We demonstrated that the CD8+CD45R+, but not the CD8+CD45R- subset of cells effected suppression following AMLR activation. In contrast, cytotoxic activity against alloantigens resided in both the CD8+CD45R+ and CD8+CD45R- subsets of cells. Biochemical analysis showed that on CD8 cells, the 220-kDa isoform of the LCA/T200 antigen family was better represented than the 200-kDa isoform, when compared to CD4 cells. The density of the CD45R antigen increased on CD8 cells following activation in AMLR and treatment of AMLR-activated CD8 cells with either anti-CD45R antibody or anti-CD3 antibody abolished the suppressor function of these cells. In contrast, treatment of AMLR-activated CD4 cells with anti-CD45R, but not anti-CD3 antibody, abolished the suppressor/inducer function of these cells. The results suggest that the CD45R antigen as well as CD3 T cell receptor complex have an important role in the suppressor function of AMLR-activated CD8 cells.