Granulocytes and monocytes derived from inflammatory exudates or the peripheral circulation of several mammalian species (1-5) can be stimulated to release a pyrogen, leukocytic pyrogen (LP) 1, which induces a characteristic febrile response in rabbits. Chemical characterization of LP has shown it to be a protein molecule (6-8) and since the original discovery of its production by rabbit peritoneal exudate cells (9), many attempts have been made to determine the mechanism of its synthesis by leukocytes. With one exception (10), preformed pyrogen has not been obtained from disrupted rabbit cells (11,12). Its release in vitro has been shown to take place over a several hour period (4, 11, 12), to be dependent upon the temperature of incubation (11), and inhibited by agents which bind sulfhydryl groups in enzymes (13). These findings suggest that LP is either synthesized de novo or activated from a precursor substance after contact with an appropriate stimulating agent.In the present studies, the role of RNA and protein synthesis and the energy requirements for pyrogen release by human peripheral leukocytes activated by the ingestion of heat-killed bacteria have been investigated. The results reported here indicate that both transcription of messenger RNA and its translation by ribosomes into