80 patients with seropositive MG at 18 academic centers will be randomized to 3 months of treatment with 2.5 gm MM/day (1,250 mg q 12 hours, +/- 2 hours) plus 20 mg prednisone/day versus placebo plus 20 mg/day prednisone. The primary measure of efficacy will be the change from baseline in Quantitative MG (QMG) score at the end of 3 months. Secondary outcome measures include survival analysis for treatment failure, MG-related impairment of daily activities, functional assessment, manual muscle testing, SF-36 Health Status, and serum concentration of antibodies to the acetylcholine receptor. Study completers will have the option of taking open-label MM for an additional 6 months, during which prednisone will be reduced to the lowest dose necessary to maintain the optimum clinical response.