The roles of helper and suppressor T cells in the development and expression of antibody responses to GAT were studied in (responder x responder)F1 mice immunized with parental GAT-MΦ. Spleen cells from B10 x B10.D2)F1 mice primed in vivo with B10 or B10.D2 GAT-MΦ developed secondary in vitro plaque-forming cell (PFC) responses only when stimulated by GAT-MΦ syngeneic with the GAT-MΦ used for in vivo priming. By contrast, virgin F1 spleen cells developed comparable primary PFC responses to both parental GAT-MΦ. Co-culture of T cells from (B10 x B10.D2)F1 mice primed in vivo by B10 GAT-MΦ with virgin (B10 x B10.D2)1 spleen cells demonstrated the presence of suppressor cells that inhibited the primary response of virgin spleen cells stimulated by B10.D2 GAT-MΦ. Spleen cells from (B10 x B10.D2)F1 mice primed in vivo with B10.D2 GAT-MΦ had suppressor T cells that suppressed primary responses stimulated by B10 GAT-MΦ. The suppressor T cell mechanism was composed of at least two regulatory T cell subsets. Suppressor-induced T cells were Lyt-2-,I-J+ and must be derived from immune spleen cells. Suppressor-effector T cells can be derived from virgin or immune spleens and were Lyt-2+ cells. When the suppressor mechanism was disabled by treatment with 1000 rad gamma irradiation or removal of Lyt-2+ cells, Lyt-2- helper T cells from (B10 x B10.D2)F1 mice primed with B10 GAT-MΦ provided radioresistant help to virgin F1 B cells stimulated by B10 but not B10.D2 GAT-MΦ. Suppressor inducer Lyt-2-,I-J+ cells from B10 GAT-MΦ-primed (B10 x B10.D2)F1 mice were separated from the primed Lyt-2-,I-J- helper T cells. In the presence of Lyt-2+ suppressor effector cells, the Lyt-2-,I-J+ suppressor-inducer suppressed the primary response of virgin spleen or virgin T plus B cells stimulated by both B10 and B10.D2 GAT-MΦ. Therefore, suppressor T cells were able to suppress primary but not secondary GAT-specific PFC responses stimulated by either parental GAT-MΦ. These results showed that immunization of (responder x responder)F1 mice with parental GAT-MΦ results in the development of antigen-specific helper and suppressor T cells. The primed helper T cells were radioresistant and were genetically restricted to interact with GAT in association with the major histocompatibility complex antigens of the MΦ used for in vivo priming. Suppressor T cells were radiosensitive, and their expression of function was not genetically restricted but their activity contributed to the observed restriction by suppressing primary PFC responses stimulated by the parental GAT-MΦ not used for in vivo priming.