Plasmapheresis combined with prednisone and azathioprine therapy produced striking clinical improvement in five patients with myasthenia gravis who still had moderate to severe disability despite thymectomy, high-dose prednisone therapy and optimal doses of cholinesterase inhibitors. Serial determinations of titers of serum antibody toward the acetylcholine receptor demonstrated a fall to 21 +/- 5 per cent (mean +/- S.D.) of the original levels concurrently with the patients' increasing strength. Clinically improved patients maintained lowered titers, whereas clinical relapses were associated with a rebound in titer. Our results suggest that plasmapheresis will find a place in the management of patients with myasthenia gravis, and they implicate antibodies to acetylcholine receptor as a pathogenic factor in this disease.