The GAT-specific suppressor T-cell factor (GAT-TsF) extracted from lymphoid cells from GAT-primed, nonresponder DBA/1 mice has been partially characterized. It is a protein that has affinity for GAT and determinants encoded by the I region of the H-2 complex. On the basis of specificity and avidity, GAT-TsF resembles anti-GAT-MBSA antibodies produced by DBA/1 mice in spite of the fact that it is too small to be classical antibody and has no constant-region determinants of heavy or light chains. Further, GAT or a fragment of GAT is associated with the GAT-TsF. GAT-TsF has been partially purified from the crude extract by absorption to GAT-Sepharose and elution with 0.4 to 0.6 KCl. GAT-TsF purified on the basis of its affinity for GAT bears I-region determinants but not detectable GAT or GAT fragment.