Inflammatory reaction caused by intramuscular injections of turpentine in 5 rabbits led to an obvious increase of not only plasma fibrinogen and factor VIII: C levels, but also of plasma antithrombin III activity, measured by a chromogenic assay as heparin cofactor. This activity rose from 80% +/- 10,8 (mean +/- SEM) before the injection to 123% +/- 6,56 48 hours later. Changes affecting plasma fibrinogen level and antithrombin II activity were much lesser in a group of 5 rabbits given small doses of intravenous endotoxin. It is considered that acute inflammation is accompanied by the setting of the hemostatic balance at a higher level.