Interleukin-10 is produced during incubation of human whole blood with bacterial lipopolysaccharide (LPS) and down-regulates tumour necrosis factor-alpha production in this in-vitro model of endotoxaemia. 39 out of 69 (57%) patients with gram-negative (n = 25) or gram-positive septicaemia (n = 44) had increased plasma interleukin-10 (range 12-2740 pg/mL), whereas interleukin-10 was undetectable in 29 out of 33 control patients without infection and in 20 healthy volunteers. Patients with septic shock (n = 21) had higher interleukin-10 (main 58 pg/mL) than septicaemic patients without shock (11 pg/mL, p < 0.001). We conclude that interleukin-10 is produced during sepsis and might be involved in the control of the inflammatory response induced by bacterial products.