Constituents other than factor IX have been implicated as etiologic agents for thrombotic complications in patients receiving prothrombin complex concentrates (PCCs). In vitro studies, in vivo animal models, and clinical evaluations in patients with hemophilia B indicate that high-purity factor IX concentrates contain significantly fewer potentially thrombogenic contaminants than PCCs. A recent in vitro study from our laboratory used highly sensitive assays to analyze the relative purity of these newer products. The following products were studied using sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) analysis and immunoblotting: Aimafix, AlphaNine-SD, Factor IX VHP, Immunine, Mononine, Nanotiv, and 9MC (now known as Replinine). The mean specific activity of the high-purity factor IX products ranged from 68 IU factor IX/mg (Aimafix) to 246 IU factor IX/mg (Mononine). SDS-PAGE analysis under reducing and nonreducing conditions showed that Mononine had the fewest contaminating bands. The immunoblot to detect factor IX showed a dominant factor IX band for all products, visible light chain of factor IX for all products except Aimafix, and another contaminating band visible at 49,500 daltons for all products except 9MC. High molecular weight contaminants were apparent for some products. Factor Vila was detected in some lots of Immunine, Nanotiv and 9MC. Factor X and prothrombin contaminated Aimafix, AlphaNine-SD, Factor IX VHP, Immunine, Nanotiv and 9MC. Thus, Mononine, Nanotiv and 9MC demonstrated the highest purity but no product was totally free of contaminants.