The concentrations of IgG subclass antibodies (Ab) to acetylcholine receptor (AchR) were quantified in 36 patients with myasthenia gravis (MG) treated with pyridostigmine only, and in eight patients who underwent thymectomy, using an IgG subclass-specific immunoprecipitation assay. IgG1, IgG2, IgG3, and IgG4 subclass Ab to AchR were present in 100%, 33%, 64% and 39% of the pyridostigmine-treated patients, respectively. The concentration of IgG1 Ab increased significantly with disease severity as graded by the Osserman-Genkins classification (rs = 0.37, P less than 0.05). IgG1 and IgG3 subclass protein concentrations were significantly higher (P less than 0.0003) in the 36 pyridostigmine-treated MG patients than in 44 age- and sex-matched healthy subjects. Thymectomy induced an appreciable reduction in anti-AchR IgG1 concentration in two patients, whereas six patients showed no changes in Ab to AchR. The results support the hypothesis that binding of anti-AchR IgG1 and IgG3 on AchR in the neuromuscular junction followed by complement-mediated cell lysis or phagocytosis, may play a role in the pathogenesis of MG.