An immunoadsorbent was prepared by coupling rabbit and human antibodies against human factor VIII to Sepharose CL-2B. The resulting insoluble antibodies completely removed factor VIII and fibrinogen from normal human citrate plasma. Other coagulation factors were satisfactorily recovered in the eluted plasma. Following addition of fibrinogen,the factor VIII-deficient plasma was used for calibration of the one-stage factor VIII assay and compared with the hemophilic plasma. Parallel straight lines were obtained in the log-log plot against VIII:C, indicating that the artificial reagent can be used as a substitute for hemophilic plasma in determination of factor VIII procoagulant activity. The immunoadsorbent can be regenerated and repeatedly used for affinity chromatography binding of factor VIII.