The examination of patients with Myasthenia gravis revealed that before thymectomy the number of T and B cells in the peripheral blood did not deviate from the norm. It was also observed that their sera contained factor stimulating T lymphocytes to the spontaneous rosette formation, theta antigen generation in xynogeneic system and hydrocortisone-resistant thymocytes. No factor inhibiting the spontaneous rosette formation by T lymphocytes or causing the transmission from Thy-1.2+ cells to Thy-1.2-cells was detected in these sera. Thymectomy resulted in the immediate fall of T lymphocytes number in the peripheral blood of the patients examined. Besides, serum of most patients almost totally lost the stimulating factor. The fall of T cells number did not involve the appearance of the inhibitory factor in the patients' serum. Different than in others results gave blood and serum analysis in the patient tymectomized 10 years earlier. Her blood contained the normal number of T and B lymphocytes and the stimulatory factor of a "limited" activity. It displayed only the ability of generating hydrocortisone-resistant cells, it appeared nonactive in generating theta antigen or restoring the ability of rosette formation by T cells.