The in vitro suppression of lysozyme (HEL)-primed helper T cell function (AgTh) is dependent on both suppressor inducer T cells (Tsi) (bearing the Lyt-1 and I-J surface antigens) and suppressor effector T cells (Tse) (bearing the Lyt-2 and I-A cell surface antigens). The AgTh and Tsi appear in draining lymph nodes of antigen-primed mice within 10 days of footpad immunization, whereas the Tse is detected in the lymph node around 21 days. Tse are localized to the spleen after i.p. immunization and represent the predominant functional T cell subset. In an attempt to understand the idiotypic relationships that exist among the three T cell types associated with anti-HEL responses in genetic low responder mice, each subset was characterized for expression of the predominant idiotype of anti-lysozyme antibodies, IdXL, by treatment with a cytotoxic anti-IdXL antibody and for IdXL-specificity by binding to IdXL-coated plates. Only the Tse was found to bear IdXL. The Tsi, on the other hand, binds to IdXL, and the AgTh belongs to a distinct idiotypic universe. These results suggest that the mode of communication between the Tsi and Tse is via idiotypic complementarity.