Endotoxin was as a pathogen to investigate its effects on bronchial hyperreactivity formation. The latent periods of guinea pigs inhaled histamine and PD2 value of isoproterenol were markedly decreased 4 days after the intraperitoneal injection of endotoxin 1 mg/kg, meanwhile PD2 value of histamine and histamine levels of lung tissue and plasma were increased significantly. These results indicate that endotoxin is a very important factor in the infection induced bronchial hyperreactivity.