A murine cytotoxic monoclonal antibody (GF 22.1) was produced by Balb/c immunization with GRA human lymphoblastoid cells. HLA-A, B and C blanketing and SDS-PAGE analysis of the immunoprecipitate demonstrated MHC class I structures as the targets. Cytotoxic assays were performed with peripheral blood lymphocytes from 84 unrelated donors and from members of 15 families at different antibody dilutions. Statistical analysis was performed by Fisher test on each dilution separately and by Mann-Whitney U test on the dilutions taken all together. The data suggest the detection of a cross-reacting group (B15, A32, B17, B40/w41 and B21) with high affinity and the inclusion of other antigens (B12, B35, A2, B13, A11 and A24) with a lower affinity.