Priming of BALB/c mice with phosphorylcholine-hemocyanin (PC-Hy) induces T helper cells that are detected in splenic fragment cultures responding to immunization with trinitrophenylated PC-binding myeloma proteins, TEPC 15 (TNP-T15) and MOPC 167 (TNP-M167). Trinitrophenylation did not alter the binding site, idiotype, or isotype of the antibodies as demonstrated by binding studies. To assay idiotype-recognizing helper cells, Ly-2.2-depleted T cells from PC-Hy-primed donor mice were transferred to syngeneic athymic mice. Splenic anti-trinitrophenol fragment cultures were prepared from the nude recipients, and the response to TNP-T15 and TNP-M167 was measured by enzyme-linked immunosorbent assay. The number of responding fragments is dependent on the number of transferred primed T cells. The homing efficiency of 51Cr-labeled helper cells into the spleen of nude recipients was determined. The frequencies of T helper cells taken from PC-Hy-primed donors required for a B cell response to TNP-T15 or TNP-M167 were indistinguishable. The fine specificity of the anti-PC idiotype-recognizing T helper cells was studied by adding hapten (PC) or unconjugated myeloma proteins to fragment cultures as inhibitors at the time of immunization. PC and PC-bovine serum albumin, as well as T15 and M167, inhibited the helper function in vitro. Furthermore, free heavy chains of T15 and M167 partially inhibited T help, but free light chains of both idiotypes had no effect. These findings collectively show that T helper cells, induced by priming with antigen, recognize a shared idiotypic determination on T15 and M167 that is part of the PC binding site. The heavy chains of T15 and M167 appears to be the major structural component of this determinant. Evidently, T helper cells can recognize a shared determinant that is present on idiotypically different myeloma proteins. This determinant appears to be conserved throughout evolutionary and somatic mutations. The role of this shared, binding site-related idiotypic determinant as a regulatory idiotype in T-B cell interaction is discussed.