In cases of epidemic influenza the microbe is found in the mucus discharged, and it is more pure the further down the bronchial tree the discharge has been produced. In cases of death pure cultupes have been found in the fine bronchi. In the febrile stage the bacilli are mostly free, but during convalescence they are chiefly in the substance of leucocytes.

It is a very small thin rod, non-motile. It is stained by the ordinary dyes but not by Gram's method.

Cultures are best made on blood-agar, on which the growth forms minute transparent drop-like colonies in about twenty-four hours. The cultures require a temperature of 26° to 42° C.

Animals are mostly refractory to the bacillus, but monkeys have been found susceptible, when cultures have been injected into the trachea. The toxine produces marked symptoms of poisoning in the rabbit.