Changes of basophils in peripheral blood were observed in 201 patients with bronchial asthma. And this study is concerned with the threshold of asthmatic attack indicated by basophils and the relationship between basophils and reagin-type antibody-antigen reaction. The results show that the basophils increase before the attack in an episode of asthmatic attack. The threshold of asthmatic attack indicated by basophil count of 65/cmm is very significant for the observation of asthmatic attack, and it is possible to expect a next attack in the pre-attack stage, if the basophil tends to increase over this value. And this threshold of asthmatic attack was constant in individual cases. These changes of basophils were similar in the cases given steroid hormone. In eosinophils these changes were not so clear-cut. In a prolonged therapeutic course basophils in bronchial asthma were shown over the threshold in the stage of repeated asthmatic attacks, and lower in the non-attack stage reflecting the therapeutic effect. It was also found that basophils increased clearly in patients with sea-squirt asthma who showed positive Prausnitz-Küstner reaction. This result indicates that there is a close relation between basophils and reaginic antibody-antigen reaction. The action of heparin contained in basophils concerned with the mechanism of allergic reaction of reagin-type was examined. And it was observed that heparin suppressed the allergic reaction caused by reaginic antibody in the rat, and was an antagonist for histamine. Therefore, it was considered that one of the roles of basophilic leukocytes is to repair the reaction of reagin-type allergy.