Antigen-specific T cell suppressor factor (TsF): isolation of a cDNA clone encoding for a functional polypeptide chain of phosphorylcholine-specific TsF* A cDNA library of phosphorylcholine (PC)-specific suppressor T hybridoma, 29-C-6, was constructed. By differential colony hybridization, 68 colonies were selected which hybridized with mRNA probes of two PC-specific suppressor T hybridomas, 29-C-6 and 09-M-24-8, but not with those of BW5147. A hybridization translation assay revealed that only one combination of translation products of cDNA clones, p6-1 and p6-5, showed strong PC-T cell suppressor factor (TsF) activity. Sequence analysis showed that p6-5 contained a specific cDNA sequence of about 800 base pairs (bp) while p6-1 had a 190 bp poly(A) sequence insert. When total poly(A)+RNA of 29-C-6 was hybridized with a p6-1 filter alone the recovered mRNA was capable of producing PC-TsF. However, when the total poly-(A)+RNA was hybridized with a p6-1 filter combined with a p6-5 filter, the mRNA eluted from the p6-1 filter was not able to produce PC-TsF, suggesting that the depletion of p6-5 specific mRNA from p6-1-binding poly(A)+RNA led to a complete abolishment of the capability to produce PC-TsF. Furthermore, p6-5 hybridizing mRNA could successfully restore the p6-1-binding poly(A)+RNA depleted of p6-5-specific mRNA, and translation products of both RNA mixtures showed strong PC-TsF activity. These results suggest that PC-TsF is not a single polypeptide chain, but is composed of at least two distinct polypeptide chains, and also that p6-5 contains a cDNA sequence encoding for one of the polypeptide chains composing the PC-TsF molecule.