This investigation focused on the role of adherent accessory cells and their cellular product, interleukin 1 (IL 1), in cellular immune responses associated with experimental autoimmune encephalomyelitis (EAE) in Lewis rats. Guinea pig myelin basic protein (GPMBP)-sensitized lymph node cells (LNC) responded in culture with GPMBP by undergoing activation as measured by augmented transfer of EAE to syngeneic recipients, and proliferation as measured by [3H]thymidine incorporation. GPMBP-sensitized LNC, after depletion of adherent accessory cells, no longer responded to GPMBP in the EAE transfer activation assay. In contrast, aliquots of the same LNC preparation exhibited proliferative responses to GPMBP that were only partially reduced. Addition of irradiated thymocytes to adherent cell-depleted cultures fully reconstituted responsiveness to GPMBP in the activation assay and restored full reactivity to GPMBP in the proliferation assay. Furthermore, addition of either purified human IL 1 or recombinant human IL 1 to adherent cell-depleted cultures reconstituted reactivity to GPMBP in the EAE transfer activation assay and augmented GPMBP-specific proliferative responses. Anti-Ia monoclonal antibodies blocked GPMBP + IL 1-induced cellular activation of nonadherent LNC. These results demonstrate that both IL 1 and Ia molecules are important in the pathway leading to GPMBP-induced activation of EAE-inducing T lymphocytes. Furthermore, these results suggest that different accessory signals may be required for optimal induction of GPMBP-induced lymphocyte activation vs GPMBP-specific proliferative responses.