The synthetic terpolymer of L glutamic acid 60 L alanine30 L tyrosine10 (GAT) not only fails to elicit a GAT specific antibody response in nonresponder mice, but also prior injection of GAT specifically decreases the ability of nonresponder mice to develop a GAT specific antibody response to a subsequent challenge with GAT MBSA. This inhibition is mediated by GAT specific suppressor T cells. Further, a suppressive factor can be extracted from lymphoid cells of GAT primed nonresponder mice that inhibits the development of primary GAT specific antibody responses to GAT MBSA and to GAT PRBC by normal syngeneic mice. The suppressive activity is dose dependent and absorbed by GAT Sepharose, but not by BSA Sepharose. The suppressive activity elutes from a G 100 Sephadex column in the same fraction as ovalbumin, suggesting its m.w. is approximately 45,000 daltons.