Human monocytes and neutrophils were separated from buffy coats of blood obtained from normal donors. Following incubation with heat-killed staphylococci, monocyte preparations contained 20 times more pyrogenic activity in the supernatant media than did supernates from an equal number of neutrophils. During purification of these pyrogens it was discovered that these cell preparations each produced a distinct and different pyrogen. The pyrogen obtained from neutrophils had a mol wt of 15,000 following Sephadex G-75 gel filtration, an isoelectric point of 6.9, and could be precipitated and recovered from 50% ethanol at –10°C. In contrast, the pyrogen derived from monocyte preparations had a mol wt of 38,000, an isoelectric point of 5.1, and was destroyed in cold ethanol. Both molecules were unaffected by viral neuraminidase but biologically destroyed at 80°C for 20 min and with trypsin at pH 8.0. The febrile peak produced by partially purified neutrophil pyrogen occurred at 40 min while that from monocytes was at 60 min. In addition, monocyte pyrogen produced more sustained fevers for the same peak elevation as neutrophil pyrogen. These studies demonstrate for the first time two chemically and biologically distinctive pyrogens derived from circulating human white blood cells and have important implications for our understanding of the pathogenesis of fever in man.