Protein C is a vitamin-K-dependent plasma glycoprotein that when activated inhibits coagulation by selectively inactivating the active forms of factor V and factor VIII. A specific antiserum to protein C has been raised, and plasma protein C levels have been measured by means of an electroimmunoassay in several physiological and pathological conditions. In 60 healthy adults there were no differences in protein C related to age or sex; protein C levels ranged from 72 to 139% of values in a normal plasma pool. Low levels were found in 12 healthy full-term newborn infants; the levels in 20 women in the last trimester of normal pregnancy were no different from those in healthy non-pregnant women. In 58 patients with chronic liver diseases protein C levels were lower than those in healthy subjects, in degrees roughly proportional to the severity of the disease. Protein C levels were very low in 21 patients with the disseminated intravascular coagulation syndrome, particularly in those who had evidence of consumption coagulopathy. Very low levels were also found, however, in 20 patients with adult respiratory distress syndrome without consumption coagulopathy. Acquired defects of protein C developed after surgery in the patients operated on for malignancies, after major abdominal operations for benign conditions, and also after relatively minor procedures such as appendicectomy and hernia repair. These findings indicate that protein C deficiencies occur in several conditions associated with increased tendency to thrombosis.