Morphological changes of basophils stimulated by anti-IgE and anti-IgG were observed in 73 asthmatic and 10 healthy subjects by smear specimen. Basophil reactivity to both anti-IgE and anti-IgG was low in the healthy subjects. Basophils from atopic asthmatics reacted to anti-IgE more strongly than the cells from the healthy subjects, while the basophil response to anti-IgG was low. Basophil reactivity of intractable asthmatics to anti-IgG was higher than the reactivity to anti-IgE. Basophils from asthmatic subjects with a high serum IgE level reacted to anti-IgE strongly, while the cells from subjects with a low serum IgE level (less than 100IU/ml) showed a high response to anti-IgG. In the cases with a basophil ratio of less than 0.5, basophils released a large amout of histamine by stimulation with anti-IgE. The release of histamine induced by anti-IgE was low in the cases with low reactive basophils. These results suggest that observation of morphological changes of basophils stimulated by anti-IgE and anti-IgG is useful for the clinical study of bronchial asthma.