With a bioassay sensitive for recombinant tumour necrosis factor at a concentration of 10(-4) ng/ml, we were able to detect tumour necrosis factor-like cytotoxicity in serum from three patients in the initial phase of septicaemia. The cytotoxic activity corresponded to a concentration of recombinant tumour necrosis factor of 2 X 10(-2)-3 X 10(-3) ng/ml serum. The concentration in cerebrospinal fluid was estimated to be three times higher than in serum. In 23 untreated patients with malignant disease of varying origin and stage, including two patients with severe cachexia, we were not able to detect tumour necrosis factor-like cytotoxicity.