In the present studies it has been confirmed that the random copolymer of L glutamic acid50 L tyrosine50 (GT) fails to induce an antibody response in a large number of inbred strains of mice. Nevertheless, GT complexed to methylated bovine serum albumin (MBSA) elicit a GT specific IgG PFC response in vivo. Furthermore, injection of BALB/c mice with 10 to 100 μg of GT specifically decreases their ability to develop anti GT PFC responses to a subsequent challenge with GT MBSA. GT specific tolerance can be transferred to normal, syngeneic recipients by spleen cells or thymocytes of GT primed animals. These results indicate that the stimulation of suppressor cells can be observed in nonresponder mice with another synthetic polypeptide besides GAT. Various parameters of GT specific immunosuppression in BALB/c mice are described. The application of these techniques to the study of the genetic factors controlling the stimulation of specific immune suppression is discussed.