The authors used a liquid-phase radiobinding assay to measure serum anti-myelin oligodendrocyte protein (MOG) immunoglobulin (Ig) G in 87 patients with multiple sclerosis (MS), in 12 patients with encephalomyelitis, and in 47 healthy subjects. Anti-MOG IgM was determined in samples obtained at onset from 40 of 87 patients with MS and in control subjects. The frequency of positive samples with low titers of anti-MOG IgG (≤5.7%) and IgM (≤8.3%) was similar in all the groups and subgroups. Binding competition experiments showed that these antibodies had low affinity. Anti-MOG antibodies are not disease specific.