A GAT-specific 'second order' suppressor T cell factor (TsF2) from the hybridoma 762 B3.7 has been purified and biochemically characterized. The protein has a m.w. of approximately 66,000, an isoelectric point of 6.8 to 6.9, and elutes from a reversed phase HPLC column in two peaks, one in 55% acetonitrile, the other in 70% propanol. Amino acid analysis of both forms gave similar molar ratios, suggesting that the two forms are closely related and many differ mainly in the degree of posttranslational modification. SDS-PAGE electrophoresis under reducing conditions gave two chains of the apparent m.w. of 42,000 and 35,000.