In eighteen patients with fulminant hepatic failure (FHF), in grade III or IV coma, both protein C antigen and activity were significantly decreased (0.35 +/- 0.03 u/ml and 0.35 +/- 0.03 u/ml respectively). There was a significant correlation between protein C antigen and activity (r = 0.61, p less than 0.01). Protein C antigen levels were inversely correlated with prothrombin time (r = -0.57, p less than 0.05) as were protein C activity levels (r = -0.57, p less than 0.05). There was also significant correlations between fibrinogen and protein C antigen (r = 0.69, p less than 0.01) and protein C activity (r = 0.61, p less than 0.01). These results demonstrate that the naturally occurring inhibitor of coagulation, protein C, is present at low levels in FHF and this is probably due to the lack of synthesis of the protein in the damaged liver. The low levels of protein C may make these patients more susceptible to the disseminated intravascular coagulation which is known to occur in FHF and this in turn will lead to a further reduction in protein C levels.