The immunogenic properties of HLA-A9 antigens isolated from serum have been evaluated. A9 antigens at various stages of purification can elicit the formation of cytotoxic antibodies which become operationally specific to A9 either after absorption of the xenoantisera with cultured human lymphoid cells or human red blood cells, or after dilution of xenoantisera with human serum. A9 xenoantisera do not affect mixed lymphocyte reactions between allogeneic lymphocytes carrying A9, suggesting that coating of antigens of the A locus does not impair the functional activity of lymphocytes in the mixed lymphocyte reaction.