Fractionation of a human antibody to factor VIII: Ag by immunoabsorption with factor VIII/phospholipid (PL) complex has produced two pools of labelled Fab' fragments which can be used in fluid-phase immunoradiometric assays (IRMAs). One pool binds only to the PL-binding sites on factor VIII:Ag (and thus measures only that factor VIII:Ag in a sample not bound to PL), while the second binds to other antigenic sites. Parallel assays of factor VIII-containing materials using these two pools provide estimates of the proportion of factor VIII: Ag bound to PL in those materials. Five batches each of nine brands of factor VIII concentrates from eight different manufacturers were tested for PL-bound factor VIII:Ag by this method: all contained substantial amounts, ranging from 28% to 54% of the total factor VIII:Ag present. In addition, varying amounts of the factor VIII:Ag present in both non-activated and activated prothrombin complex concentrates (PCCs) from four manufacturers were PL-bound.