In vitro and in vivo characteristics of cryoprecipitates and three commercial factor VIII concentrates (Kryobulin, Hemofil and Koate) were comparatively studied. Factor VIII:C/VIIIR:Ag ratio was very low in all commercial concentrates without differences among them. Conversely, the decrease of factor VIIIR:WFRCof was proportional to the degree of purity. Factor VIII:C/VIIIR:WFRCof ratio was shown to be a reliable index of factor VIII complex denaturation. Crossed electroimmunoassay showed a faster migration of factor VIIIR:Ag only in commercial concentrates. Using a two-compartmental open model that accounts for endogenous synthesis of factor VIII:C, single-dose kinetics of factor VIII:C were studied in 23 patients with classic hemophilia. Good agreement between measured and fitted values of factor VIII:C plasma concentration was observed. beta half-life was shorter in high purity concentrates and longer in intermediate purity concentrates and cryoprecipitates.