The use of human whole blood as an in vitro model for detection of compounds with the potential to induce fever is described. As a readout, mediators (endogenous pyrogens) released from leukocytes such as tumor necrosis factor (TNF), interleukin I (IL-I), interleukin 6 (IL-6) and prostaglandin E2 (PGE2) are quantitated. In fact, minute concentrations of pyrogens induced the release of mediators in amounts that were easily detectable. Endotoxins as well as other bacterial components induce this reaction. We used the antipyrogenic drug aspirin to show that in blood of volunteers the ex vivo stimulated PGE2 release was inhibited for several hours. This model seems superior to the rabbit pyrogen test or the Limulus assay for endotoxin. We propose to evaluate this system as an alternative in vitro method for pyrogen testing.