The ability of spleen cells from (responder X nonresponder)F(1) mice immunized with various GAT-Mφ, GAT-MBSA, and soluble GAT to develop IgG GAT-specific PFC responses in vitro after stimulation with responder and nonresponder parental and F(1) GAT-Mφ, was investigated. F(1) spleen cells from mice immunized with F(1) GAT-Mφ or GAT-MBSA developed secondary responses to responder and nonresponder parental and F(1) GAT- Mφ, but not to unrelated third party GAT-Mφ. Spleen cells from F(1) mice immunized with either parental GAT-Mφ developed secondary responses to F(1) GAT-Mφ and only the parental GAT-Mφ used for immunization in vivo. Soluble GAT-primed F(1) spleen cells responded to F(1) and responder parental, but not nonresponder parental, GAT-Mφ. Simultaneous immunization in vivo with the various GAT-Mφ or GAT-MBSA plus soluble GAT modulated the response pattern of these F(1) spleen cells such that they developed secondary responses only to F(1) and parental responder GAT-Mφ regardless of the response pattern observed after immunization with the various GAT-Mφ or GAT-MBSA alone. These observations demonstrate the critical importance of the physical state of the GAT used for immunization in determining the subsequent response pattern of immune F(1) spleen cells to the parental and F(1) GAT-Mφ. Further, suppressor T cells, capable of inhibiting primary responses to GAT by virgin F(1) spleen cells stimulated by nonresponder parental GAT-Mφ, were demonstrated in spleens of F(1) mice immunized with soluble GAT, but not those primed with F(1) GAT-Mφ. Because responder parental mice develop both helper and suppressor T cells after immunization with GAT-Mφ, and soluble GAT preferentially stimulates suppressor T cells whereas GAT-Mφ stimulate helper T cells in nonresponder parental mice, these observations suggest that distinct subsets of T cells exist in F(1) mice which behave phenotypically as responder and nonresponder parental T cells after immunization with soluble GAT and GAT- Mφ.