DAP12 is an ITAM-bearing membrane adaptor molecule implicated in the activation of NK and myeloid cells. In mice rendered DAP12 deficient by targeted gene disruption, lymphoid and myeloid development was apparently normal, although the activating Ly49 receptors on NK cells were downregulated and nonfunctional. To analyze the consequences of DAP12 deficiency in vivo, we examined the susceptibility of DAP12^−/− mice to experimental autoimmune encephalomyelitis (EAE). DAP12^−/− mice were resistant to EAE induced by immunization with myelin oligodendrocyte glycoprotein (MOG) peptide. Resistance was associated with a strongly diminished production of IFNγ by myelin-reactive CD4⁺ T cells due to inadequate T cell priming in vivo. These data suggest that DAP12 signaling may be required for optimal antigen-presenting cell (APC) function or inflammation.