The E variant of encephalomyocarditis (EMC) virus causes an encephalomyelitis and coagulative necrosis of the pancreas and parotid glands in some but not all strains of inbred and outbred mice. In other models of disease caused by picornaviruses, depletion of specific lymphocyte subsets abrogates the development of tissue lesions. In this study, severe encephalomyelitis and acinar pancreatitis and parotitis developed in adult male A/J mice infected with 100 PFU of EMC virus. Depletion of the CD4+ subset of T lymphocytes in vivo with a monoclonal antibody (MAb) prior to EMC virus inoculation protects mice from developing encephalomyelitis, pancreatitis, and parotitis. This effect is also seen when animals are treated with anti-CD4 and anti-CD8 in combination, but the anti-CD8 MAb alone does not ameliorate the disease. Overall, concentrations of virus in tissues from anti-CD4-treated animals are lower than in immunologically intact control mice. Small-plaque variants of virus were also recovered from the tissues in some animals in this group. CD4+ lymphocytes are involved in the expression of EMC virus-induced pancreatitis and parotitis in A/J mice. This specific subset of T cells would appear to influence EMC viral tropism or replication in various organs.