Homozygous protein C (PC) deficiency is a rare genetic defect that usually results in fatal thrombotic complications (purpura fulminans and DIC), but it can be successfully managed with oral anticoagulants or PC replacement. The successful use of PC replacement for two individuals is described. The activity and antigen levels of PC in fresh frozen plasma (FFP) and prothrombin complex concentrate (PCC) are also reported. The concentration of PC in FFP is 87 +/- 15 units/dl. PC is present in all PCC analyzed; however, a ten-fold difference between the various brands and/or lots is noted. The PC activity and antigen correlates well with no significant levels of APC. Upon infusion of FFP into two homozygous PC-deficient children, the PC levels obtained were less than or equal to 30 units/dl post-infusion and undetectable after 12-18 hr. With infusions of PCC, plasma levels of PC obtained were 100-145 units/dl and less than 10 units/dl after 48 hr. The percent recovery and half-lives of PC from FFP and PCC were 49.8% and 7.8 hr, and 84% and 7.4 hr, respectively. One infant was treated every 48 hr for 2 years without significant purpura fulminans or DIC complications. The levels of the other PC system components did not change during the infusion of the PC-rich material. Based on this information, a specific replacement protocol has been developed using a PC-rich concentrate. However, several problems may arise with the "less pure" PC-rich concentrates: catheter-tip thrombosis, related large vessel thrombosis and blood-transmitted diseases. With a specific PC concentrate, replacement therapy is a viable alternative for the long-term management/treatment of homozygous PC deficiency.