Leukocytic pyrogen previously reported to contain an essential protein moiety, appears to be a lipid-protein complex having a molecular weight in the range of 10,000 to 20,000. Evidence that it contains essential lipid includes its inactivation by Cu(++), its lability in alkaline solutions (pH 8.5 and above), and its loss of pyrogenicity when extracted with acid-isooctane. Its solubility in 66% methanol, and the enhancing action of ethanol in freeing it from sonicated cells, suggest the presence of exposed lipid groups at its surface. Once the complex is separated from other proteins, its biological activity is readily destroyed. Although the lipid component is presumed to contain unesterified fatty acid(s), its precise composition is unknown. The finding of lipid in the active complex is in keeping with the hypothesis that the pyrogen is derived from leukocytic membranes.