Distributions of peripheral blood lymphocyte subpopulations and T-cell subsets were studied in 38 patients with myasthenia gravis (MG) and 23 healthy controls. T cells were detected by rosette formation with sheep red blood cells and B cells with erythrocyte-antibody-complement (EAC) complexes. T mu cells were identified by rosette formation of T cells with Ox RBC-IgM complexes, and T gamma cells with Ox RBC-IgG complexes. The means of total lymphocyte count and active T cell percentage were marginally significantly lower in MG patients than in normal controls (0.05 less than P less than 0.1). The mean percentage of T mu cells was higher in MG patients than in controls (50.2 +/- 11.8% vs. 38.5 +/- 15.3%, P less than 0.01), whereas that of T gamma cells in the former was lower than that in the latter (20.4 +/- 7.1% vs. 27.6 +/- 6.2%, P less than 0.001). Thus, the loss of regulatory suppressor T cells and the increase of helper T cells in the patients are in favour of regarding MG as an autoimmune disease. The fact that the percentage of T gamma cells was significantly decreased in MG patients after autologus serum incubation, and that no such a phenomenon was seen in normal controls, may suggest a possible role of serum factors in the pathogenesis of MG.