We have investigated changes in serum interleukin 6 (IL-6) in patients undergoing elective cholecystectomy. Serum IL-6 increased in all patients within 1.5 hour of incision, reaching a maximum between 1.5-4 hours after incision (median 50 U/ml; range 22-79 U/ml). The maximum serum IL-6 correlated with the length of the operation (r = 0.95). Serum C-reactive protein was not detectable until 8-12 hours post-incision, but maximum serum C-reactive protein did not correlate with maximum serum IL-6 concentration or length of operation. There was no consistent increase in plasma interleukin 1 or tumour necrosis factor following surgery. Serum IL-6 is an early marker of tissue damage and may be of value in the study of the metabolic response to injury.