Sixty patients with myasthenia gravis were examined prospectively by measuring serial titers of antibodies against human acetylcholine receptor, and these were correlated with a quantitative clinical score. Serial titers of antibodies detected by the standard immunoprecipitation assay (binding antibodies) correlated with the clinical score in most patients. Antibodies blocking the binding of alpha-bungarotoxin to receptors (blocking antibodies) were detected in 29 patients. Serial blocking antibody titers correlated with changes in muscle weakness less often than binding antibody titers. Titers of both classes of antibodies often followed a divergent course, suggesting that the autoimmune B-cell clones that formed these classes of antibodies may have been activated asynchronously.