The addition of an antihuman immunoglobulin (AHG) reagent to the basic complement-dependent cytotoxicity (CDC) test markedly increases the frequency of lymphocyte-reactive antibodies in many alloantisera. The extra reactivity has been previously identified as associated with alloantigens coded by the HLA complex, but definitive evidence establishing the antigenic specificity of the antibodies reactive by AHG-CDC has been lacking. We determined the nature and specificity of AHG-reactive alloantibodies through parallel testing (CDC +/- AHG) of a large battery of HLA alloantisera against panel cells composed of unrelated individuals and genotypic HLA-identical sibling pairs, and by means of differential platelet absorption and elution of alloantibody. We conclude that the AHG-CDC procedure, relative to "standard" CDC, detects subthreshold levels of alloantibody with specificity for the HLA-A, B, and C locus alloantigens. Most importantly, the AHGG-CDC technique consistently converts cytotoxicity-negative absorption-positive (CYNAP) HLA alloantibody to direct cytotoxic antibody, thus providing a more accurate assessment of the complete specificity of antibodies in complex alloantisera and patients' sera without having to resort to more cumbersome binding assays. These data should be of assistance in improving the characterization of HLA alloantisera used for serological and biochemical studies of the HLA molecules and in delineating the specificity of AHG-CDC antibody in clinical allotransplantation and single-donor platelet transfusion.