The in vitro properties of 5 factor VIII preparations (AHF‐Kabi, Hemofil Hyland, AHF‐Profilate Abbott, Kryobulin Immuno and Factorate High Purity Armour) and an ordinary cryoprecipitate were studied with reference to factor VIII clotting activity (VIII:C), factor VIII clotting antigen (VIILCAg), factor VIII related antigen (VIIIR:Ag) (EI, IRMA, CIE), ristocetin cofactor activity (VIIIR:RCF), fibrinogen and factor XIII. All the preparations with the exception of Factorate had higher levels of VIII:CAg than VIII:C indicating inactivation of the biological activity of VIII:C during the procedure. AHF‐Kabi (fraction I‐0) and the cryoprecipitate, the only preparations capable of normalising the defect in patients with von Willebrand's disease, showed the same level of VIIIR:Ag determined by EI and by IRMA, while all the other preparations (i.e. cryoprecipitates purified further in different ways) had considerably lower levels of VIIIR:Ag determined by IRMA than by EL Based on these in vitro techniques it seems to be possible to predict which preparations can be used successfully in patients with von Willebrand's disease, while no such conclusions can be made from VIIIR:RCF determinations.EI yielded similar concentrations of factor XIIIasubunit in all the preparations tested. 3 functional assays showed high factor XIII activities in AHF‐Kabi but low or no activities in the others.Thus, considerable differences were found of the in vitro properties of the proteins in 5 factor VIII concentrates and a cryoprecipitate. The action of proteases and the techniques used in the purification procedure are probably of crucial importance for the properties of the various factors.