Injection of extracts or suspensions of various rabbit tissues was found to be without effect upon the body temperature of normal rabbits. Occasionally, extracts of bone marrow produced transient fever, and saline extracts of acute inflammatory lesions of the Shwartzman and Arthus types were found to produce fever when injected intravenously. Suspensions or extracts of polymorphonuclear leukocytes collected from sterile peritoneal exudates contain a heat-labile substance which produced fever whereas those of erythrocytes, macrophages, and lymphocytes failed to do so. The cell-free supernatant fluid of sterile peritoneal exudates obtained at the stage when polymorphonuclear leukocytes predominate also proved capable of producing fever.