Plasma exchange produces a short-term clinical improvement in myasthenia gravis (M.G.) which may be attributable to removal of acetylcholine receptor (AChR) antibody. The possibility that repeated plasma exchanges might confer cumulative long-term benefits was investigated. Serum-AChR-antibody and clinical response were followed for 4--12 months (mean 8 months) in six M.G. patients receiving 4--25 plasma exchanges of 2--4 1 together with immunosuppressive drugs (azathioprine [2.5 mg/kg] with or without alternate-day prednisone therapy), and in seven M.G. patients on immunosuppressive drugs alone. Percentage decrease in AChR antibody was not significantly different in the two treatment groups. Decline in antibody titre was associated with clinical improvement. Eight patients with previous thymoma showed significantly greater decline in antibody than the remaining five patients, irrespective of plasma exchange. Since repeated plasma exchange had no cumulative long-term benefit, the value of this treatment as used here lies only in short-term control of severe M.G. symptoms.