Immunization of BALB/c mice with phosphorylcholine-Limulus polyphemus hemocyanin (PC-Hy) induces a population of T cells that recognize the predominant PC-binding antibody, TEPC15 (T15). The splenic fragment culture system was used to examine the specificity of these T cells for a series of PC-binding myeloma and hybridoma antibodies representing the prototype variable region of the heavy chain (VH)T 15 sequence as well as somatic variants of the T15 germ line-encoded sequence. Included in this group of PC-binding proteins were both T15-positive and T15-negative antibodies, as defined by anti-idiotypic antibody. T cell help was identified by the ability to promote TNP-specific B cell responses to trinitrophenylated PC-binding proteins. It was found that T cells generated by immunization with PC-Hy recognize both antibodies with the T15 prototype sequence and the putative somatic variants of this sequence. A population of these T cells appear to recognize common determinants shared by these proteins because immunization with T15 itself also induces the recognition of the somatic variants. This suggests that idiotopes encoded in the T15 germ line gene expressed by the T15 prototype idiotype and the somatic variants can function as targets for T cell recognition and are thus regulatory idiotopes.