Genes mapping in the I region of the H-2 complex control a system of lymphocyte alloantigens (Ia) which are expressed on a subpopulation of T cells and on most B cells. Specific anti-Ia serum in the presence of rabbit complement removed the splenic T-cell subpopulation responsive to Con-A, but did not affect the response to phytohemagglutinin (PHA) or Leucoagglutinin. Antibodies specific for Ia, H-2K, or H-2D membrane antigens were used without complement to pretreat spleen cells. These antibody pretreated cells responded normally to Con-A and PHA.