Because of the restricted heterogeneity of idiotypes generated in the course of antiphosphorylcholine (PC)* immune responses and the availability of idiotype-specific probes, we studied idiotypic and anti-idiotypic murine T cells in the PC system. PC-specific helper activity was assayed in mice expressing varying levels of humoral HOPC-8 (H8) idiotype and inhibited with both conventional anti-H8 antiserum produced in A/He mice and a monoclonal anti-H8 reagent obtained from a hybridoma. Our results using the conventional anti-H8 indicate that T cell receptors bearing H8 idiotypic determinants are produced in the absence of circulating H8 antibody, and that H8 idiotype expression by T and B cells in the same mouse may not show parallel expression of idiotypes. The monospecific anti-H8 Ig could suppress H8-positive B cell responses but was incapable of inhibiting PC-specific T cells. The implications of these findings are discussed. Furthermore, we demonstrate that immunization with purified anti-PC idiotype generates T cells which regulate the proliferation of idiotype-bearing myeloma cells. © 1980.