The antibody and T cell proliferative responses to Staphylococcal nuclease, a small peptide antigen, have been shown to be under Ir gene control. In addition anti-idiotypic antisera have been produced with specificity for anti-nuclease antibodies of a number of mouse strains. It has been demonstrated that these idiotypic markers are linked to allotype markers but not to MHC encoded (H-2) determinants. The present study has investigated the cellular expression of idiotypic markers in this Ir gene controlled response. In vitro augmented primary responses to TNP-nuclease could be generated after priming with nuclease (antigen), anti-idiotypic antisera (Ab2), or anti-(anti-(anti-idiotype)) (Ab4). These responses to TNP-Nase were shown to be inhibited by anti-idiotype present in culture. It was demonstrated that antigen, anti-idiotype or Ab4 primed T cells were required and that treatment of the primed T cells with anti-idiotype and complement abrogated their helper cell activity. Thus, direct evidence for helper T cell expression of idiotypic markers has been obtained although it is as yet unclear whether the T cell synthesizes or acquires these cell surface determinants. Studies with nuclease specific T cell clones are underway to further elucidate the nature of helper T cells and their antigen-specific receptors.