Twenty-one patients with arthritis and dermatitis following intestinal bypass surgery were studied. The arthritis was polyarticular, remittent, and intermittent. Typically, the synovial fluid was inflammatory. The commonest inflammatory skin lesion was a vesiculopustular dermatitis. Nineteen patients had serum immune complexes using the Raji cell technic. Seventeen patients had serum cryoproteins, primarily consisting of IgG 1, IgG 3, C3, and C4. Three patients had both Escherichia coli antigens and anti-E. coli antibody in their cryoprotein. Five patients had granular and one had linear deposits of immunoglobulin and complement at the dermoepidermal junction. Further evidence that bacterial antigens play a role in tissue injury was provided by detection of granular deposits of E. coli antigen at the dermoepidermal junctions in two patients, and at the glomerular capillary basement membrane in one patient.