The role of endogenous mediated fever and exogenous hyperthermia as modulators of immunological processes against infection and neoplasms remains poorly understood. Hyperthermia appears to modulate the activity of a number of immunological cytokines including Interleukin-1 (IL-1). Using a recently developed Radiant Heat technology for safely producing 41-42 degrees C Whole Body Hyperthermia in mice, we have investigated the effect of 1 hour of 41 +/- 0.5 degrees C WBH on the production of cutaneous IL-1. After a transient fall in IL-1 production 4 hours post WBH, significant elevations (up to twice control levels) were noted with a peak at 16-20 hours post WBH. IL-1 levels remained elevated for 5-6 days post-WBH. The potential clinical significance of this finding is discussed with particular reference to human malignant melanoma.