The action of IgG antibody on basophils was studied during hyposensitization therapy in bronchial asthmatics. IgE and IgG antibodies bound to the surface of basophils, and the redistribution of IgE receptors were observed by immuno-scanning electron microscopy. The ratio of IgG to IgE antibodies bound to basophils increased significantly during hyposensitization therapy (p<0.01). The occurrence of cap and patch formations of IgE receptors on the surface of basophils diminished during hyposensitization therapy. The effects on the passive sensitization of basophils with IgG(4) myeloma protein were also studied in vitro. The results showed that the ratio of IgG(4) to IgE antibodies on the cell increased, and binding of anti-IgE antibody significantly diminished (p<0.01). The percent of reactive basophils decreased when anti-IgE antiserum was added. These findings indicate that IgG(4) antibodies affect the binding between IgE antibody and anti-IgE or antigens on the surface of basophils as a blocking antibody in hyposensitization therapy.