Rabbit xenoantisera against membranes of mouse spleen cells and cultured human lymphocytes were used to monitor the purification of solubilized mouse H–2 and human HL–A antigens in parallel with routine serological methods employing alloantisera. Antigenic material was subjected to gel filtration, ion exchange chromatography and electrophoresis in polyacrylamide gel, and in all cases the only material which inhibited the lymphocytotoxic action of the xenoantisera was that which inhibited defined alloantisera. Conversely, all alloantigenic material inhibited xenoantisera. Highly purified H–2 and HL–A preparations completely inhibited the lymphocytotoxic action of xenoantisera to crude membrane antigen. The specificity of the anti‐mouse xenoantisera was examined. Antibodies specific for the immunizing mouse strains were revealed only after so‐called species‐specific antibodies were removed by judicious absorption of the serum with cells of a mouse of another H–2 type. The residual strain‐discriminating antibodies reacted with target cells from a large panel of different mouse strains with an intensity directly proportional to the number of reacting H–2 specificities. It was not possible, however, to detect monospecific H–2 antibodies in any absorbed sera It is concluded that the lymphocytotoxic antibodies in xenoantisera predominantly are directed against determinants intimately associated with H–