It was recently suggested that chemical mediators such as leukotrienes and PAF are more important in the pathogenesis of bronchial asthma than histamine. In this study, to clarify the role of PAF in bronchial asthma, I measured PAF activity derived from human neutrophils by the platelet aggregation tachnique and evaluated this activity in relation to the clinical features of brochial asthma. Neutrophils were separated from heparinized venous blood of 34 bronchial asthmatics and 11 healthy subjects. After suspension at 6×10(6) cells/ml, the cells were treated with 5μg/ml Ca ionophore A23187 for 5 minutes. PAF was extracted from the cell suspension using chloroform, and the activity was measured with an aggregometer using rabbit platelets. PAF activaty was significantly higher (p<0.05) in asthmatics (8.3±6.9 pmol/1×10(6) cells) than in healthy subjects (4.1±3.2 pmol/1×10(6) cells), and tended to increase in steroid-independent asthmatics rather than in steroid-dependent asthma patients. However, there was no difference in PAF production between atopic and non-atopic asthmatics, or between the early and late onset asthmatics. PAF production was increased in patients with severe airway hypersensitivity. These finding suggest that PAF plays an important role in the development and exacerbation of asthma.