Serum IgE level was measured by the methods of radioimmunosorbent test (RIST) in 116 subjects including 41 patients with malignant lymphomas and 20 patients of myasthenia gravis. In Hodgkin's disease, the serum IgE level was evaluated according to the clinical stages and to the effects of chemotherapy. Also, the changes of serum IgE level was compared before and after thymectomy in myasthenia gravis patients. The results were as follows: 1) Patients with Hodgkin's disease showed remarkably high IgE levels (1726±2180 u/ml). But, serum IgE level in patients with reticulum cell sarcoma, lymphosarcoma, multiple myeloma except IgE myeloma, leukemia and lung cancer was below the normal value. 2) Most of Hodgkin's disease patients with stage Ⅲ showed high IgE level. 3) Hodgkin's disease patients with mixed cellularity according to Rye's classification showed higher IgE level than other histological types. 4) IgE level in most of Hodgkin's disease patients decreased after chemotherapy. The rebound of IgE level was remarked after cessation of chemotherapy in some of the complete remission patients. 5) The background of allergic disposition was negligible in Hodgkin's disease patients with high IgE level. Most of the patients showed negative reactions of PPD skin test and low blastoid reactivity of lymphocytes by phytohemagglutinin, but the level of IgG, IgA and IgM was not significantly variable. These results suggest that the impaired regulatory functions due to T cells might be a cause of high IgE level in Hodgkin's disease. 6) In 6 cases of myasthenia gravis with proliferative thymoma, the temporary rebound phenomenon of IgE level in 2 months after thymectomy and the gradual decrease of IgE level after 18 months were characteristic. The simultaneous observations of T and B cells suggested that initial phenomenon might be due to the loss of suppressor T cell in the thymus and the second phenomenon might be due to the decrease of helper T cell in the peripheral lymphoid tissues.