A hybridoma-derived, GAT-specific suppressor T cell factor (GAT-TsFR) from responder C57BL/10 mice has been purified to apparent chemical homogeneity using reversed phase HPLC techniques. 40 l of starting material yielded approximately 880 micrograms protein with a specific activity of 28.4 X 10(3) S50 U/ng protein representing a purification factor of 4.2 X 10(6). Purified GAT-TsFR is a hydrophobic protein with a minimum molecular weight of 18,000 that is capable of forming biologically active aggregates with molecular weights of 28,000, 64,000 and approximately 84,000 and has a pI of 6.4. GAT-TsFR is a glycoprotein that binds GAT and GT, but not GA, and bears determinants encoded by the I-J subregion of the H-2 complex. This GAT-TsFR derived from an H-2b responder haplotype to GAT is compared with GAT-TsF derived from the nonresponder H-2q haplotype on the basis of biochemical and some serological properties.