Recent studies indicate that histocompatibillty-linked immune response (It) 1 genes, are expressed on thymus-derived (T) lymphocytes. In the mouse, the genetically controlled difference in the IgG antibody response between high and low responders immunized with branched-chain amino acid polymers is lost in thymus-deprived mice (1). In guinea pigs, the initiation of functions thougtrt to be regulated by T lymphocytes (delayed cutaneous hypersensitivity, antigen-induced lymphocyte proliferation in vitro, and carrier function) requires the presence of the appropriate Ir gene (2). Thus, guinea pigs that lack the poly-L-lysine (PLL) gene (nonresponders) fail to develop either delayed hypersensitivity or other cellular immune phenomena upon immunization with 2,4-dinitrophenyl (DNP)-poly-L-lysine. Nonetheless, these animals are capable of recognizing DNP-PLL as a hapten and produce antibodies to DNP-PLL when this molecule is coupled to an immunogenic protein carrier (3). In addition, recent data indicate that the number of specific antigen-binding bone marrow-derived (B) lymphocytes does not differ between responders and nonresponders (4-6). Schlossman (7) has also recently shown that the antibody clones and the variable region gene pool for antibodies to a-DNP-deca-L-lysine, an antigen the response to which is controlled by the PLL gene, appear to be indentical in responder and nonresponder animals. Thus, the weight of experimental evidence suggests that the main functional role of the Ir gene product is in the process of antigen recognition by the T lymphocyte.However, an alternative explanation to these studies is that the Ir genes might be expressed in a cell that controls and regulates T-cell activation, i.e., the macrophage. A number of attempts have been made to resolve this issue by using cell transfer studies (8, 9). When spleen and lymph node cells were transferred from (2 X 13)F1 animals to lethally irradiated strain 13 guinea pigs that had been reconstituted with syngeneic bone marrow, a large percentage of the recipients developed the ability to respond to antigens controlled by the PLL gene; lethally irradiated strain 13 guinea 1 Abbreviations used in this paper: DNP, 2,4-dinitrophenyl; GL, a copolymer of L-glutamic acid and L-lysine; GT, a copolymer of L-glutarnic acid and L-tyrosine; It, immune response; LNL, lymph node lymphocytes; NGPS, normal guinea pig serum; PEC, peritoneal exudate cells; PELs, peritoneal exudate lymphocytes; PHA, phytohemagglutinin; PLL, poly-L-lysine; PPD, purified protein derivative of tuberculin.