To observe the participation of heparin in asthma attacks, plasma concentrations of heparin were estimated in asthmatic subjects. Changes in the plasma concentration of heparin after intravenous administration were also examined during asthma attacks. The mean plasma concentration of heparin was 0.017u/5ml in healthy subjects, and 0.017u/5ml in the non-attack stage, 0.018u/5ml in the pre-attack stage and 0.016u/5ml in the attack stage of bronchial asthma patients. The plasma concentration of heparin in the mild cases of asthma was higher in the attack-free period than in the attack period, while the plasma concentration of heparin in the severe cases of asthma was significantly lower in the attack-free period than in the attack period. There was a significant correlation between the plasma concentration of heparin and the peripheral basophil count. The time-dependent reduction in plasma heparin after intravenous administration was more remarkable in asthmatic subjects than in healthy subjects, and the reduction was most remarkable in the large attack stage of asthma. The half-reduction time of the plasma heparin concentration after intravenous administration was 1h and 20min.