This report deals with the results of the reactions obtained on mixing antivaccinia serum from the rabbit with extracts of material from the lesions of vaccinia and smallpox. It appears to be established definitely that the resulting flocculation or precipitation is specific and that variola and vaccinia are identical immunologically. The technic of preparing the extracts for the test and the method of making the test are described in detail. The possibilities of new methods of immunization by means of mixtures of antiserum and virus are discussed. The report will be of especial interest to all investigators of fundamental problems of variola and vaccinia.