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Twenty-three lots of five antithrombin III (AT III) concentrates from four manufacturers were analyzed in a single-blind study. All the preparations had been virus-inactivated by pasteurization, and one concentrate had also been treated with solvent/detergent (S/D). AT III activities were determined using two thrombin-based and one factor Xa-based chromogenic substrate assays. AT III antigen was measured by kinetic nephelometry. All AT III assays were tested against the first international reference preparation coded 72/1. In addition, AT III was characterized by crossed immunoelectrophoresis in the presence of heparin and by gel filtration. The following were quantified: heparin cofactor II activity and antigen content, heparin activity, thrombin-AT III complexes, AT III-protease complexes, total protein, albumin, immunoglobulins, glucose and pH. The AT III concentrates differed markedly in terms of their purity and potency. The specific activities of AT III and the ratios of AT III activity to antigen content ranged from 3.4 to 6.9 and from 0.63 to 0.84, respectively. The highest values were found in five lots of the concentrate that had been treated by both pasteurization and S/D. This preparation was the only one that was virtually free of denaturated AT III, as judged by crossed immunoelectrophoresis. Marked batch-to-batch variation in AT III potencies was found in two out of the five preparations analyzed. In two out of five lots from one manufacturer, the measured potencies were more than 10% lower than the declared potencies.(ABSTRACT TRUNCATED AT 250 WORDS)