The suppressor T-cell hybridoma 1556A2.1 can be induced by the monoclonal L-glutamic acid60-L-alanine30-L-tyrosine10 (GAT)-specific suppressor inducer 372B3.5 and soluble GAT to synthesize a disulfide-linked heterodimeric protein (GAT-TsF2), which directly suppresses a primary in vitro immune response to GAT. Induction and synthesis of the GAT-TsF2 protein is correlated with the appearance of specific mRNA, as detected by translation in vitro in a wheat germ cell-free extract of RNA isolated at various times after induction. The mRNA coding for the polypeptide chain that bears a serologically defined I-J determinant (I-J+ chain) appeared 8 hr after induction, whereas the mRNA coding for the antigen-binding chain (AB+ chain) was not detected until 16 hr after induction. The mRNAs coding for the individual chains sedimented as different species, suggesting that the two-chain factor is the product of two genes. The AB+ chain of the 1556A2.1 GAT-TsF2 was synthesized on membrane-bound polysomes, whereas the I-J+ chain was translated on free polysomes. The AB+ chain was synthesized from two independent mRNA species sedimenting at 10 S and 28 S, whereas a single 16S mRNA encoded the I-J+ chain. The in vitro translated I-J+ chain was bound by a monoclonal antibody against the I-J+ determinant of only the appropriate H-2 haplotype. These results suggest that posttranslational modification, including glycosylation, is not required for biological activity or for expression of the I-J epitope on the GAT-TsF2 molecule.