The exact nature of humoral immunity in virus infections and its relation to the establishment of various types of lesions in such infections are still not well understood. From a practical standpoint it would be most desirable to clarify this relation, since specific serum might be of great value in the prevention and treatment of a considerable number of diseases, particularly those of childhood. It is well known that in the case of certain virus diseases, particularly measles, an extremely effective passive immunity may be obtained by the administration of comparatively small doses of specific convalescent serum, and this procedure is in extensive use at present as a prophylactic measure. In the case of other virus diseases, especially those of which the infecting agent is inoculated directly into the tissues, there is still considerable doubt as to the efficiency of the specific serums. Vaccinia represents such a virus disease and,