For a better understanding of the molecular nature of the antigen-specific T-cell recognition system, continuous T-cell lines specific to the synthetic polypeptide antigen poly(Tyr,Glu)poly(DLAla)--poly(Lys) [(T,G)-A-L] were established from C3H.SW (high-responder) activated T-cells, cloned, and characterized. These lines and their derived clones are also constitutive secretors of antigen-specific T-cell replacing helper factors. The secreted T-cell helper factor was shown to possess MHC determinants as well as V-region determinants, or more specifically, idiotypic determinants that are cross-reactive with those expressed on (T,G)-A-L-specific antibodies of the same mouse strain. Using the fluorescence-activated cell sorter (FACS 11) and individual C57BL/6 anti-idiotypic sera produced against (T,G)-A-L-specific antibodies of C3H.SW origin, we have demonstrated the expression of the cross-reactive idiotypic markers on the monoclonal helper T-cells. Attempts were made to purify the active fraction of the T-cell factors secreted by the (T,G)-A--L continuous helper lines. Gel analysis of the twice affinity-purified eluate of a (T,G)-A--L column revealed the existence of iodinated bands with molecular weight of 17,000 and 15,000, in addition to a diffuse band of high molecular weight. The specific helper activity of the factors was associated with a 65-7570 ammonium sulfate precipitate. Gel electrophoresis of the latter fraction, as well as of an eluate of a (T,G)-A-L-Sepharose column indicated that a high-molecular-weight (<67,000) and a low-molecular-weight (15,000-17,000) fraction contained the biological activity of the factor. Similar results were obtained following chromatography of the factor on Sephadex G-100 columns. The two fractions were shown to be synthesized by the T-cell lines, as indicated by internal labeling experiments using "5S-methionine. Thus, it is suggested that a fraction of an apparent molecular weight of 15,000-17,000 preserves both the antigen specificity and the helper activity of the factor produced by the (T,G)-A--L-specific T-cell lines.