Macrophages activated at sites of tissue injury produce interleukin-1, which induces hepatocytes to synthesize acute phase proteins (APP). Daily serum levels of C-reactive protein (CRP), haptoglobin (HPT), transferrin (TRF), alpha-1 antitrypsin, and ceruloplasmin (CER) were measured in 60 patients, 30 having inguinal herniorrhaphy (H), 18 cholecystectomy (C), and 12 major abdominal trauma (MAT). APP response was proportional to the level of tissue injury. CRP rose in all groups, MAT greater than C, which was greater than H. HPT levels were depressed in MAT, presumably due to removal of hemoglobin-HPT complexes from the serum. TRF was severely depressed in MAT and may be implicated in the higher infection susceptibility in this group. CER was elevated in C, suggesting a stimulating mechanism in this group as opposed to H and MAT. Explanation for this is unknown. APP changes, especially CRP, may be useful as markers of the amount of tissue damage.