Blood basophils in subjects with bronchial asthma migrate from blood stream into local allergic reaction sites after inhalation of antigen into airways. The phenomena can be observed by changes in number of basophils in the peripheral blood. The peripheral basophil count is at a normal level in non-attack stage of asthmatics as in healthy subjects. The basophil count increases in pre-attack stages, and decreases during attack stages. These findings suggest that number of basophils changes in close relation to asthma cycle, and that by observing number of basophils in the peripheral blood, it is possible to detect near future attacks. The decrease in number of basophils and histamine release by stimulation with antigen and anti-IgE can be observed in vitro in whole blood and basophils separated by density gradient centrifugation and by counterflow centrifugation elutriation. The decreased number of basophils in the peripheral blood represents migration of the cells into local allergic reaction sites.