We have studied the frequency of human retrovirus antibody (HTLV-I, II, III) in the serum and CSF of patients with MS, matched controls, and patients with optic neuritis, idiopathic and postencephalitic Parkinson's disease, neuropathies, polymyositis, ALS, and postpoliomyelitis. Except for the postpoliomyelitis samples, all samples were collected prior to 1980. Contrary to a previous published report, no significant levels of antibody to HTLV-I, II, or III were found in the MS patients or controls. No retrovirus antibody was detected in patients with the other neurologic diseases.