A murine monoclonal antibody, 12.7G3, directed against an Ia antigen encoded by genes in the I-A b subregion of the H-2 genetic complex, was found to be cytotoxic against human B lymphocytes. When tested against a random panel of normal human donors, the reactivity of 12.7G3 exhibited a correlation coefficient of 0.58–0.68 with cells expressing HLA-DR2. Antibody reactivity segregated with HLA-DR2 in two families studied. Binding of 12.7G3, as detected by immunofluorescence using flow microfluorometry, was positive for two human cell lines, GM 3161 and HFB-1, both expressing HLA-DR2, and negative for two other cell lines, GM 3104 (DR1,1) and GM 3164 (DR4,4).
