A strain of swine influenza has been observed to change from a condition of full contagiousness, in which both H. influenzae suis and the swine influenza virus were transferred by pen contact, to one of only partial contagiousness, in which the virus alone was transferred, resulting in the mild filtrate disease instead of swine influenza in animals infected by contact. Swine that had been experimentally converted into carriers of H. influenzae suis developed swine influenza following contact with animals infected with the altered strain of the disease. Experiments in which the etiological components of a freshly obtained and fully contagious strain of swine influenza were substituted for the corresponding components of the altered strain of the disease revealed the fact that the change in the contagious character of the latter was due to an alteration in the bacterial component of the etiological complex and that the virus component was in no way responsible.

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