The level of IgG was determined in the serum of patients with myasthenia using the disc precipitation test. 66 samples of sera were examined. In 24 cases (36.3%) the IgG level was 2SD above the mean value obtained in the control group of healthy subjects. The obtained results were analysed in relation to various clinical parameters. The IgG level +2SD above the mean value of the control group was found in 50% of thymectomized patients and in 12.4% of patients without indications for thymectomy. Taking into account the time from thymectomy the IgG level +2SD above normal value of the control group was found in 60% of patients 1--2 months after the operation. The lowest percent of changes (40%) was observed in patients over 5 years after thymectomy. Taking into consideration the clinical form of the disease according to the classification of Osserman the IgG level 2SD above the mean value of the control group was found in 60% of sera from patients withe the third clinical form -- acute, severe, and in only 26.4% of cases with mild form -- 2a. The serum IgG level in various age groups showed the greatest number of changes in patients past the age of 50 years (57.1%), and lowest in the group aged 10--20 years (11.7%). The IgG level 2SD above the mean value of the control group was found primarily in patients in the 1st year of the disease (42.5%).