It appears to be widely believed that the fever of infective disease is due to absorption of phyrogenetic substances liberated from dead microorganisms by a combuined process of lysis and extraction in the body of the infected subject. The diffusion of products of the action of bacterial autolytic enzymes on microorganisms after their destruction by the defensive agencies of the body is a variant on the same theme. Closely allied are the theories of protein fever, and of anaphylactic fever, in so far as they relate to the supposed possession by microorganisms, or their split products, of pyrogenetic properties. The absorption of alien protein, or of derivatives of alien protoplasm, from organisms that are dead, is, in short, the essential feature of this conception of fever production.