Factor VIII procoagulant activity (VIIIC) is exerted by a low molecular weight (LMW) moiety of the factor VIII molecule that can be separated from a high molecular weight (HMW) moiety by high ionic strength buffers. In this investigation the procoagulant activity of the LWM moiety of factor VIII prepared by immuno-adsorbent chromatography and its relationship to the HMW moiety of haemophilic plasma was studied by means of different VIIIC assay systems and using different substrates in regard to their content of the HMW VIII moiety. LMW VIIIC was prepared by immunoadsorbent chromatography; HMW VIII without VIIIC was prepared by chromatographing cryoprecipitate from a coagulant antigen negative severe haemophiliac on 4% agarose. The LMW VIIIC obtained by immunoadsorbent chromatography gave higher VIIIC values when tested in the one-stage partial thromboplastin time (PTT) system using von Willebrand's disease plasma as substrate than using haemophilic plasma as substrate. This finding was shown to be related to the HMW VIII measured as VIII related antigen (VIII: Ag) in the substrate plasmas. When the VIIIR: Ag was removed from the haemophilic substrate plasma by immunoadsorption, the VIIIC values obtained for the LMW VIIIC were higher. Also, adding HMW VIII purified from haemophilic plasma to the von Willebrand's disease substrate plasma resulted in lower VIIIC values for the LMW VIIIC in the PTT system.