1. Endogenous pyrogen (EP) was detected in tumour extracts from four patients with renal carcinoma and fever. No EP was detected in extracts of tumours from six afebrile patients or in extracts of normal kidney.
2. Tumour slices from one febrile patient released EP into a nutrient incubation medium; this process was suppressed by the presence of cycloheximide. Tumour slices from an afebrile patient did not release EP on incubation.
3. Leucocytes from the peripheral blood of two febrile patients with renal carcinoma did not yield EP on incubation in saline.
4. The production of EP by leucocytes ‘activated’ by 2 h incubation with a Proteus endotoxin was not inhibited by cycloheximide.
5. It is suggested that fever associated with renal carcinoma is caused by the release of EP within the tumour.