AbstractAn enzyme‐linked immunosorbent assay (Elisa) method was developed in order to examine prevalence and titer of antibodies directed against the factor VIII coagulant protein (F.VIII:C) in hemophilia A and nonhemophilia A patients. Highly purified F.VIII:C was used as immunosorbent on microtiter plates with a peroxidase‐conjugated goat anti human IgG antibody for F.VIII:C antibody detection. Results determined by Elisa were compared with measurements according to the Bethesda method. Initially 24 plasma samples containing an F.VIII:C inhibitory activity ranging from 0 to 7,700 Bethesda units (BU) were analysed. At plasma dilutions of 1:128 the optical density determined by our Elisa measurement and the corresponding BU showed a logarithmic correlation. The coefficient of correlation was r=0.92 with a standard deviation of 0.002 from the regression curve. Plasma samples were analysed from 53 hemophilia A patients, from 21 nonhemophilia patients with acquired F.VIII:C antibodies and from 460 randomly selected nonhemophilia patients presenting for routine preoperative coagulation examination. F.VIII:C antibody‐positive Elisa results and positive BU were found in 7 hemophilia A patients and the 2 patients with a history of acquired F.VIII:C antibodies. Positive Elisa results and negative BU were found in 1 hemophilia A patient and 25 out of 460 nonhemophilia A patients (5.43%) suggesting F.VIII:C antibodies without inhibitory potency on F.VIII:C in these cases. The Elisa method proved to be suitable for F.VIII:C antibody screening in hemophilia and nonhemophilia patients. The Elisa protocol is easy to repr