The pyrogenicity of bacteria, bacterial products, fungi and viruses is usually tested in rabbit pyrogen test. The current hypothesis on the pathogenesis of fever is as follows (1, 13, 36): First, granulocytes and mononuclear cells phagocytize the virus particles, that is the exogeneous pyrogen, and then release an (lysosomal ?) endogeneous pyrogen. This substance is identical with the ]eukocytic pyrogen and seems to act directly on the hypothalamus. Besides the endogeneous pyrogen just mentioned other biologically active compounds are also to be found in the serum (e.g. lysozym% interferon and other antibacterial or antiviral substances). After a single injection of a large quantity of virus particles complete tolerance is induced in test animals. A second inoculation of exogeneous pyrogen leads no longer to a pyrogenic reaction. An identical or similar mechanism is assumed for the action of bacterial endotoxin.The complete mechanism of pathogencsis of microbially-induced fever is poorly understood. One way of studying this problem is to investigate single steps in the development of fever. It is possible to influence a) the exogeneous pyrogen itself (e.g. by heat, chemicals or specific antibodies (32, 36) ; b) the conditions of phagocytosis (e.g. by adding cations, anions or enzymes or by varying the osmosis (1, 31) ; II*