Like most experimental autoimmune disease experimental allergic encephalomyelitis (EAE) has been shown to be mediated by CD4+ helper T cells. In vivo antibody blocking studies with anti-CD4 and adoptive transfer of activated CD4+ T cells indicate the importance of CD4+ cells in disease induction. Fourth backcross generation mutant CD4-/-PL/J mice were immunized with myelin basic protein. Despite the lack CD4+ T cells some of these mice developed EAE, albeit, at a considerably reduced frequency and with variable severity. Furthermore, antigen-specific T cell proliferation can be demonstrated, indicating some residual helper activity that is major histocompatibility complex class II restricted. This demonstrated that, although the CD4+ T cell is the prime effector cell in EAE, in mice developmentally lacking in CD4, the expanded double-negative T cells may subserve helper and effector functions.