When compared to values recorded in the 32 healthy control subjects, plasma protein C activity was found to be significantly decreased in the 29 patients with acute leukemia and especially in those considered to be in a critical condition. On the other hand, plasma antithrombin III activity did not significantly differ from the values noted in control subjects. The concomitantly occurring high plasma fibrinogen levels and low serum cholinesterase activity were highly suggestive for a switch of hepatic protein synthesis towards the production of acute phase proteins. It is therefore considered that in the absence of a consumption coagulopathy, changes affecting plasma protein C and antithrombin III should be related to a modified pattern of hepatic protein synthesis.