MOPC-104E (M104E) idiotype-specific and major histocompatibility complex-restricted T lymphocyte activities were investigated in BALB/c (Igh-1a) and its Igh-1-congenic strains of mice, such as C.AL-20 (Igh-1d), BAB-14 (Igh-1b), and CB-20 (Igh-1b). Idiotype-specific T lymphocytes could be induced in BALB/c and BAB-14 by immunization of viable M104E tumor cells followed by surgical removal and in vitro restimulation with the homologous tumor cells. On the other hand, C.AL-20 and CB-20 mice did not show the idiotype-recognizing capacity even though they could mount comparable cytotoxic T lymphocyte activities against M104E to BALB/c and BAB-14. The results strongly suggest that the inducibility of M104E cross-reactive idiotypy closely paralleled the producibility of corresponding idiotype-specific T lymphocytes. Genetically defined VH gene products might act as internal images that construct idiotype network systems.