Several inbred as well as congenic resistant strains of mice, which fail to respond to the random copolymer of L glutamic acid50 L tyrosine50 (GT), were shown to develop specific PFC responses when stimulated by GT complexed to an immunogenic carrier such as methylated bovine serum albumin (MBSA). In these studies it was found that GT preimmunization has a tolerogenic effect on the response to GT MBSA in some mouse strains; whereas in other strains of mice, GT fails to inhibit the GT MBSA response. It may therefore be concluded that immune suppression cannot account for nonresponsiveness in all cases. The development of specific immune suppression in response to GT was shown to be inherited as a dominant trait in F1 hybrids resulting from the mating of suppressor with nonsuppressor strains. This trait is, therefore, under the control of a gene or genes that have been designated as specific immune suppression gene(s) Is genes. The strain distribution of GT induced suppression demonstrates that Is genes are coded for in the H 2 complex. Furthermore, immune suppression by the two related copolymers, GT and GAT, are distinct in different strains of mice. The significance of these data for our understanding of the regulation of the immune response is discussed.