Antisera were raised in rabbits against a monoclonal antibody (McAb 103) of C3H.SW origin which is specific to the synthetic polypeptide (T,G)-A-L and was shown to express the major idiotypic determinants of conventional anti-(T,G)-A-L antibodies. Antibodies were purified and were shown in a binding assay to recognize McAb 103 as well as C3H.SW anti-(T,G)-A-L antibodies. C57BL/6 mice were immunized with the purified rabbit anti-McAb 103 (Ra 103) and their lymph nodes were studied in a proliferation assay. Proliferation was observed in the presence of both Ra 103 and (T,G)-A-L, although the latter stimulated the cells to a lesser extent, suggesting the induction in vivo of (T,G)-A-L-specific clones in low frequency. A T cell line was established from these lymph node cells. The line is kept in continuous growth in the presence of IL-2 and periodic triggering with Ra 103. A significant proliferative response was obtained with Ra 103 only. This proliferation could be almost completely inhibited by either McAb 103 or by conventional anti-(T,G)-A-L antibodies of C3H.SW origin, indicating the cross reaction between the idiotypes expressed on the T cell line and the (T,G)-A-L-specific antibodies. No proliferation could be detected in the presence of either normal rabbit IgG or rabbit anti-mouse IgG. Thus, the T cell line TId 103 allows the analysis of the role of idiotype in T cell recognition and regulation.