We have used adoptive transfer of myelin basic protein (MBP)-reactive lymphocytes in the Lewis rat model of experimental allergic encephalomyelitis (EAE) to identify stages of effector cell development and to investigate the nature of the subsequent recipient response to the transferred cells. Depending on the timing of cell collection, lymph node cells (LNC) obtained from MBP-CFA (MBP emulsified in complete Freund's adjuvant)-immunized donors may directly transfer clinical disease; however, independent of disease development, recipients of LNC develop early onset of clinical disease following immunization of the recipients with MBP-CFA, consistent with the presence of MBP-memory cells in the LNC transfer inoculum. Similarly obtained spleen cells do not directly transfer disease and do not contain MBP-memory cells (as defined by the early onset of clinical disease following MBP-CFA challenge). Spleen cells adoptively transfer clinical disease only following in vitro culture stimulation with antigen or selected mitogens. Recipients of the primary culture-derived encephalitogenic spleen cells also develop an accelerated onset of clinical disease following MBP-CFA challenge, indicative of the presence of MBP-memory cells, and are not vaccinated. Encephalitogenic T cell lines adoptively transfer clinical disease, and in most cases recipients are vaccinated to MBP-CFA-induced active disease, but remain susceptible to adoptively transferred disease. Co-transfer of encephalitogenic T cell line cells with MBP-reactive lymph node or encephalitogenic spleen cells does not alter the vaccination response. We have found that during the process of T cell line development, the vaccinating phenotype is acquired following the second antigen stimulation cycle. These studies also demonstrate that regulation induced by T cell vaccination blocks the development of effector cells from precursor cells and that such regulation is also equally effective in blocking disease development in recipients which have increased numbers of memory cells. Thus, the response to T cell vaccination, once established, is fully capable of inhibiting the development of effector cells from increased numbers of precursor/memory cells, a response that would be needed in the clinical application of vaccination-induced resistance.