1. Steady-state fever has been produced in normal volunteers by an intravenous priming injection followed by a sustaining infusion of leucocyte pyrogen. During this fever, volumes of up to 550 g blood were withdrawn, and separated into cells and plasma.
2. No febrile responses were observed when on subsequent days the cells or the plasma was re-infused.
3. These findings are compatible with the hypothesis that the inability to detect a circulating pyrogen in patients with inflammatory fever, reflects the difficulty of obtaining sufficiently large volumes of blood.
4. Transfusion of blood is unlikely to be a useful means of establishing whether circulating endogenous pyrogen is concerned in naturally occurring fever.