Résumé : Seventeen burned patients were investigated-Group I (n=10) with a mean burned area expressed as unit burn standard (UBS) of 69±24 and Group II (n=7) with a mean UBS of 23±8. Blood samples were collected immediately after admission, 6-12h after injury, during the morning and evening of day I. and then daily for 2 weeks. This prospective study demonstrated complement activation in vivo in all burned patients, measured by C3d/C3 ratio index which was not related to the extent of the burned surface. A significant protease-antiprotease imbalance, correlated to the severity of burns, was found, leukocyte elastase was increased throughout the observation period, α2-macroglobulin drastically decreased in severely burned patients, and α1-proteinase, inhibitor promptly decreased below the normal level in patients with more than 40 UBS. Finally, there was a delayed but then persistent acute-phase reactant protein response involving C-reactive protein, haptoglobin and α1-acid glycoprotein, the concentrations of which reached a plateau on days 6 or 7. © 1987.