To investigate the mechanism of the late asthmatic response (LAR) which participates in intractable asthmatic attacks, humoral and cellular components at the sites of LAR were examined by bronchoalveolar lavage fluid (BALF) obtained after bronchial inhalation of house dust allergen. BAL examination was performed 2 hours after improvement of LAR, and leukotrienes (LTs) measured by high performance liquid chromatography. The level of LTC4 was significantly higher in BALF after LAR(p<0.05), and that of LTB4 was also high at LAR. The increase in the percentage of neutrophils, eosinophils and basophils were evident in BALF after LAR compared with the level in non-attack state and after IAR. However, little LTD4 was detected in the BALF after LAR. On the other hand, the percentage of neutrophils and the LTB4 level in peripheral blood showed a peak at 3 hours after bronchial inhalation of house dust allergen, and then decreased markedly during LAR. These findings suggest that neutrophils, eosinophils, and basophils may release leukotriene-dominant chemical mediators which provoke an asthmatic attack in the late phase response.