Data obtained in lizards infected with live bacteria suggest that fever may be beneficial to their survival. An adaptive value of fever has also been inferred in mammals, but the results are equivocal. Findings that certain leukocyte functions are enhanced in vitro at high temperatures have provided a possible explanation for the alleged benefits of fever. However, serious questions exist as to whether results from experiments in ectotherms and in vitro can properly be extrapolated to in vivo endothermic conditions. Indeed, various studies have yielded results inconsistent with the survival benefits attributed to fever, and fever is not an obligatory feature of all infections under all conditions. Certainly, the widespread use of antipyretics, without apparent adverse effects on the course of disease, argues against fever having great benefit to the host. In sum, although fever is a cardinal manifestation of infection, conclusive evidence that it has survival value in mammals is still lacking.