Progress in the purification of viruses has resulted, principally in the preceding decade, in an encouraging accumulation of knowledge of the chemical, physical and morphological characters of these agents. Though the initial successful studies in this field were made on bacteriophage and with agents responsible for animal diseases, widespread interest was given first to the findings with certain of the plant viruses. Work with the animal viruses under more complicated conditions for examination has continued, however, until, at the moment, there is a better understanding of some aspects of the characters of the animal viruses than of those of the agents causing disease in plants. Many general reviews (216, 87, 215, 222, 258) have been written of the findings with purified viruses, concerned in most instances with agents responsible for disease in plants. Smadel and Hoagland have reviewed (202) in detail the results with vaccinia virus and Pirie (168) has described the recent work with animal viruses.