Using mainly our improved method of Kovacs, Changes of basophilic leucocytes in peripheral blood were observed in 181 patients with bronchial asthma and 20 patients with allergic occupational asthma caused by inhaling the body fluid of sea spuirts attaching to the shell of cultured Oyster. This study shows that basophils have a strict relationship to asthmatic attacks ; the basophils show a normal level in non-attack stage as in healthy controls, an increase in pre-attack, and tend to decrease in attack. A close observation on changes in the numbers of peripheral basophils almost make an early detection of the forth-coming attack possible. The threshold of asthmatic attack indicated by basophil count, 65/cmm, is very significant for the observation of forth-coming asthmatic attack, and this threshold of asthmatic attack varied, in some extent, though constant, in each case. These changes of basophils were similar in the cases given corticosteroids. In eosinophils changes were not so clear-cut. Concerning a prolonged therapeutic course, basophils in bronchial asthma were over the threshold in the stage of repeated asthmatic attacks, and lower in the non-attack stage. Patients with allergic occupational asthma showed these similar changes.