To study the humoral reactivity in chronic myelogenous leukemia (CML), the author examined immunoglobuline levels, immune adherence hemagglutination test (IAHA) and lymphocytotoxicity test on the 24 patients admitted to our clinic. The following data were obtained: 1) The 19 patients, whose serum immunoglobuline levels were examined, were all within normal range with mean values of IgA 203±74mg/dl, IgG 1463±275mg/dl and IgM 182±67mg/dl. 2) IAHA was positive in 4 of 24 patients (16.7%) and the reactivity was correlated with the number of peripheral leukemic cells used as antigen; it was positive in 3 of 9 patients (33.3%) with leukemic cells over 4×10(4)/μ 1, but in only one of 15 patients (6.7%) with leukemic cells below 4×10(4)/μ 1. The IAHA test was also correlated with the disease state; none of 4 patients (0%) during blastic crisis of CML showed a positive reaction to their own leukemic cells. 3) Sera from 12 of 17 patients (70.6%) showed a positive lymphocytotoxicity to normal lymphocytes. In its correlation with the disease state, it was positive in 8 of 13 patients (61.5%) during remission and in all the patients (100%) during relapse. From these results it could be concluded that humoral immunity in CML is not so impaired through the clinical course except for blastic crisis.