To calrify the role of platelets at the site of allergic reaction, ultrastructural observation, measurement of plasma serotonin levels and serotonin-releasability of platelets after various stimuli were studied in 29 asthmatics and 6 normal controls. The results were as follows : 1) Plasma serotonin levels were higher during the attack stage than non-attack stage in asthmatics. 2) Morphological changes and the aggregation of platelets were observed by scanning electron microscopy after stimulation with platelet activating factor(PAF), CaI, anti-IgE and anti-IgG. 3)The serotonin-releasability of platelets stimulated by CaI was significantly decreased in severe asthmatics, compared with mild asthmatics and normal subjects(p<0.05). However, there were no significant differences between steroid dependent and non-steroid dependent intractable asthmatics in severe asthma. 4) The releasability of serotonin was decreased in most cases during asthmatic attack and was suppressed by aminophyllin. 5) The serotonin-releasability of platelets stimulated by anti-IgG of Candida antigen was remarkably enhanced aftre incubation with serum, although releasability was not increased with heart-treated serum. These results suggest that platelets play an important role in bronchoconstriction induced by various immunological reactions involved with complement activation in the pathogenesis of bronchial asthma.