Interferon (IFN) is known to be released in viral infection. In addition to the direct effect of inflammation of the respiratory tract, the released IFN is also suspected to be an important factor in the aggravation of bronchoconstriction in bronchial asthma. The effects of IFN on morphological changes of basophils from round to pear shape and histamine release from basophils were analyzed to clarify the role of IFN in bronchial asthma. IFN alone did not affect these parameters in either healthy subjects or asthmatic patients. As a model of the basic bronchial asthma reaction, anti-IgE was added to basophils with and without IFN preincubation. The IgE mediated reaction was enhanced with IFN in basophils from atopic and non-atopic asthma patients. Furthermore, the enhancement of histamine release was observed in atopic asthma, more significantly than in non-atopic asthma. The IgG mediated reaction of basophils is considered to play a role in some asthmatic patients. IFN apparently increased reactive basophils of non-atopic asthma patients in the IgG mediated reaction. On the other hand, the histamine release from basophils was so minimal that the effect of IFN on the IgG mediated reaction was not clearly detected. While the mechanism of enhancement is still controversial, the suppressive effect of histamine release by isoproterenol in addition to anti-IgE suggest the regulation of reactive basophils by the cyclic AMP concentration.