Injection of mice with L-glutamic acid50-L-tyrosine50 (GT)- or L-glutarnic acid60-L-alanine30-L-tyrosine10 (GAT)-specific suppressor T-cell factor (GT-TsF or GAT-TsF) up to 5 wk before antigenic challenge suppresses GT-methylated bovine serum albumin (MBSA) and GAT-MBSA plaque-forming cells responses. T suppressor cells are responsible for the suppression induced by the suppressive extract as demonstrated by adoptive transfer and sensitivity to anti-Thy-1 and complement treatment. We conclude that suppressive extract induces specific suppressor T cells. The material responsible for generation of suppressor T cells is a product of the I subregion of the H-2 complex. We have excluded that suppressive quantities of antigens are present in the extract. A/J mice, which can neither be suppressed by GT nor make GT-TsF can be suppressed by BALB/c GT-TsF. Spleen cells from BALB/c GT TsF-primed A/J mice can adoptively transfer suppression to normal syngeneic recipients. A/J mice appear to be genetically defective in cells involved in factor production. These results are discussed in the light of a two-step model for induction of antigen-specific suppressor cells. © American Society for Clinical Pathology.