Mouse adenovirus type 1 (MAV-1) infection of B-cell-deficient and Bruton's tyrosine kinase (Btk)-deficient mice resulted in fatal disseminated disease resembling human adenovirus infections in immunocompromised patients. Mice lacking B cells or Btk were highly susceptible to acute MAV-1 infection, in contrast to controls and mice lacking T cells. To our knowledge, this is the first demonstration that mice with an X-linked immunodeficiency phenotype (Btk deficient) are susceptible to virus-induced disease. Mice lacking B cells or Btk on a C57BL/6 background succumbed with encephalomyelitis, hepatitis, and lymphoid necrosis. Mice lacking B cells on a BALB/c background succumbed with enteritis and hepatitis. Survival of acute MAV-1 infection correlated with early T-cell-independent neutralizing antibody and T-cell-independent antiviral immunoglobulin M. Treatment of MAV-1-infected Btk(-/-) mice 4 to 9 days postinfection with antiserum harvested 6 to 9 days postinfection from MAV-1-infected Btk(+/+) mice was therapeutic. Our findings implicate a critical role for B-cell function in preventing disseminated MAV-1 infection, particularly production of early T-cell-independent antiviral immunoglobulin M.