Histamine release from basophils induced by anti-IgE was examined in 14 asthmatic subjects with blood eosinophilia. The subjects were divided into two groups; group I (with blood eosinophilia, positive skin test and positive RAST to house dust) and group II (with blood eosinophilia, negative skin test to various allergens and negative RAST to house dust). 1. Serum IgE levels in group I were much higher than those in group II. 2. Maximum percent histamine release induced by anti-IgE was much higher in group I than in group II. 3. Dose-response curve of anti-IgE-induced histamine release in group I showed consistent increase as anti-IgE concentrations increased. while, dose-response curve in group II was very low. These findings suggested that blood eosinophilia in group I might be elicited relating to IgE-mediated reaction. However, mechanism causing eosinophilia in group II was not clear in the present study.