Clinical situations with release of proteinases from blood cells or tissues into the circulation may result in a marked decrease of blood proteinase inhibitor content which in turn may result in a capillary leak syndrome, shock and even in exitus letalis. Replenishment of blood proteinase inhibitors is of benefit in such situations. In this study the inhibitory potential of fresh plasma, fresh frozen plasma and the liquid plasma protein preparation Biseko has been tested with the following enzymes: human leukocyte elastase, human plasmin, human matrix metalloproteinase-9, bovine trypsin, bovine chymotrypsin and rat liver lysosomal cathepsins. The concentration of the blood protein preparations resulting in 50% inhibition of constant amounts of each of the enzymes has been determined by plotting residual activity vs. log of concentration of blood protein preparation in enzyme assays. From these IC50 values inactivation ratios for 1 mg and 1 ml of the blood protein preparations was calculated. These inactivation ratios show an equal suitability of fresh plasma, fresh frozen plasma or full plasma proteins for replenishment of plasma proteinase inhibitory potential in vitro. As additional finding, commercial preparations of human serum albumin exert a surprisingly high inhibitory potential to lysosomal cathepsins.