Activated protein C is a potent anticoagulant which inactivates Factors V and VIII in plasma. Normal plasma contains an inhibitor of activated protein C. A previous report from our laboratory hypothesized that the absence of this inhibitor in plasmas from patients with combined Factor V/VIII deficiency could be the molecular basis for this disease. This report demonstrates the presence of protein C inhibitor in those Factor V/VIII deficient plasmas originally studied. Freezing and thawing significantly reduced the ability of normal and Factor V/VIII deficient plasmas to inhibit activated protein C. It is suggested that this explains the conflicting literature reports describing functional assays of protein C inhibitor in Factor V/VIII deficient plasma. This observation also emphasizes that extreme care must be used in handling and storing plasma samples that are to be assayed for protein C inhibitor.