An alloantiserum, TK2, obtained from a kidney transplant patient, was investigated. TK2 reacted with B lymphocytes that had only one detectable DQ antigen. DQ "blank" homozygous cell lines were positive for TK2, but DQwl, DQw2, and DQw3 homozygous cell lines failed to react with TK2. Positive reactions for TK2 segregated with DQ "blank" haplotypes in the families studied. The reactivity of TK2 to B lymphocytes was strong, while its reactivity to monocytes was very weak. When TK2 was absorbed with DQ "blank" cells, it became unreactive to other lymphocytes. TK2 blocked the binding of an anti-DQ monoclonal antibody to DQ "blank" cells. It did not affect the binding of the antibody to DQw1 and DQw3 cells, nor did it affect the binding of an anti-DR monoclonal antibody to a DQ "blank" cell line. These findings suggest that TK2 can detect a new HLA class II alloantigen that corresponds to a DQ "blank" specificity.