In the present study treatment of sepsis in 18 surgical patients, 9 survivors and 9 fatal cases, were evaluated by determining components of the plasma proteolytic enzyme systems using chromogenic peptide substrate assays. During persistent sepsis, continuous low values for prekallikrein, plasminogen and antithrombin III were found until death. At autopsy a septic focus was found in all but one of the fatal cases. Very low levels of prekallikrein during sepsis and reduced functional inhibition of plasma kallikrein in septic shock indicated a poor prognosis. In the survivors all parameters returned towards normal range upon successful therapy. Plasminogen and antithrombin III were most rapidly normalized. It is concluded that determination of components of the plasma protease systems using chromogenic peptide substrate assays, gives valuable information about course and prognosis in surgical sepsis, and that they are suitable for practical clinical use.