Alloreactive cytolytic T cell clones were generated from mixed leucocyte reactions (MLRs) between unrelated individuals. Two clones exhibited proliferative responses specific for class I HLA antigens B21 and B17. They were also found to be cytolytic toward cells bearing these HLA-B antigens as measured in cell-mediated lympholysis (CML) assays. Four clones exhibited primed lymphocyte test (PLT) and CML activity specific for various class II HLA antigens, namely DR1, DR5, DQw3 and DRw53. For each of these six clones, the CML specificity was identical to the PLT specificity. Both class I and class II specific clones released interleukin-2 (IL-2) upon restimulation with irradiated cells carrying the relevant HLA specificity. This stimulator-induced IL-2 release showed the same specificity pattern as that observed in the PLT assay. Monoclonal antibody (mAb) inhibition studies on alloreactive T cell clones showed similar inhibition profiles of PLT, CML and IL-2 release assays. These findings suggest that cytolytic activity, secondary proliferation and IL-2 release by alloactivated T cells may be induced by the same HLA encoded determinant.