The serum activities of adenosine deaminase (ADA) and its isozymes (ADA 1 and ADA 2) were measured in 31 patients with myasthenia gravis (MG). As compared with 50 normal controls, in MG the total activity and ADA 1 were significantly high (p < 0.01, respectively), and ADA 2 tended to be high. The total activity and ADA 2 were higher in generalized MG than in ocular MG. ADA 2 was significantly higher in grade IIB patients as compared with grade I patients divided by Osserman's classification (p < 0.05). Patients with bulbar signs showed a significantly high total ADA (p < 0.05) with an increasing tendency for ADA 2. There was a significant elevation of the total activity and ADA 2 in patients with positive anti-acetylcholine-receptor antibody as compared to those of the negative antibody (p < 0.05). It was concluded that total ADA, reflecting the increase of both ADA 1 and ADA 2, is high in MG; and the measurement of ADA 2 is more important because ADA 2 was increased with advancing clinical MG grade.