The relation between morphologic changes of human basophils and IgE receptors on cells from patients with bronchial asthma was studied during the reaginic hypersensitivity reaction. Peripheral blood basophils were separated and reacted with rabbit IgG anti-human IgE antiserum. Goat anti-rabbit IgG antiserum was then derivatized to carboxylate-modified latex particles, 0.254 μm in diameter, (immunolatex) and this was used as a probe to detect IgE receptors on basophil surface. Using this probe, significantly more IgE receptors were detected on the basophils from patients with atopic asthma (with high serum IgE levels) than from normal subjects and patients with intractable asthma (with normal IgE levels). Moreover, the density of the IgE receptors was much higher on pear-shaped basophils than on spherical basophils. These pear-shaped basophils are believed to be in the degranulating state during the reaginic reaction. The distribution of immunolatex particles on the basophil surface was either diffuse or displayed patch and cap formation. Patch and cap formation was more common in pear-shaped cells and patients with atopic asthma. These phenomena were temperature-dependent. These results support the view that the pear-shaped form is the functionally active form of basophils in reaginic hypersensitivity.