Experimental autoimmune encephalomyelitis (EAE) is an experimentally induced demyelinating disease mediated by CD4+ T cells specific for various myelin proteins including myelin basic protein (MBP) and myelin proteolipid protein (PLP). Although myelin- and other CNS-specific antibodies are produced in EAE, B cells and antibodies are thought by most not to play a decisive role in the induction of EAE. In this report we show that B cells serve as the major antigen-presenting cells (APC) during the T cell activation stage in lymph nodes, and that MBP-specific antibodies can greatly enhance the induction of EAE. The role of B cells as APC is demonstrated in B cell-depleted mice. EAE cannot be induced by antigen/complete Freund's adjuvant immunization unless these mice are locally reconstituted with B cells prior to immunization. The enhancing effect of antibodies is demonstrated in experiments in which EAE is induced by the adoptive transfer of encephalitogenic T cells. The adoptive transfer of large numbers of encephalitogenic T cells induces EAE in 90% of normal recipient mice, but only 33% of B cell-depleted mice get EAE at the same cell dose. The efficiency of EAE induction in B cell-depleted mice can be enhanced if MBP-specific antibodies are simultaneously administered. A similar enhancement is also seen in normal mice when the number of adoptively transferred T cells is limiting. We propose that MBP-specific antibodies enhance the presentation of myelin-derived antigens by APC in the CNS to the adoptively transferred encephalitogenic T cells.