Eosinophil cationic protein (ECP) was measured in blood serum of 15 atopic asthmatics during early (EAR) and late (LAR) asthmatic reaction triggered by specific allergen provocation. Nonspecific bronchial hyperreactivity was evaluated in histamine provocation test before and 48 hours after the allergen challenge. We observed dual asthmatic reaction (DAR) in 8 and an isolated EAR in 7 patients. The ECP serum level and nonspecific bronchial hyperreactivity were significantly higher (p < 0.05) in the DAR group when compared to EAR responders. An inverse correlation between PC20 for histamine and ECP level was shown before and after the allergen challenge in all examined subjects (R = -0.5472, p < 0.05).