Antibody to the nicotinic acetylcholine receptor is detectable in most patients with myasthenia gravis and is responsible for the defect in neuromuscular transmission present in the disorder. We evaluated the effects of thymectomy and steroid therapies on antireceptor antibody titers. The thymectomy-treated group (consisting of juvenile and early adult-onset patients) experienced a variable change in postoperative antibody titers, ranging from a 28% increase to an 84% decrease, with the mean change of an 18.3% decrease. The steroid-treated group consisted of late adult-onset myasthenics. In these patients there was a decrease in antibody titer ranging from 29% to 83%. The average time required for a 50% reduction in titer was 4.85 months and was associated with a marked clinical improvement or complete remission.