The size of human factor VIII-related protein in plasma was determined by sodium dodecyl sulfate (SDS) glyoxyl agarose electrophoresis. The protein was immobilized after the electrophoresis by coupling it to the modified agarose. It was identified by autoradiography using purified rabbit anti-factor VIII-related antigen (VIIIR:Ag). A series of multimeric forms was identified with Mr [relative MW] of 0.85-12 .times. 106. The distribution of VIIIR:Ag multimers was the same in heparin and citrate anticoagulated plasmas and in serum: the pattern was the same after freezing as in plasma kept at 37.degree. C from the time of venipuncture until the electrophoresis was complete. VIIIR:Ag apparently circulates in normal plasma as a population of very large multimers; the size distribution is not an artifact induced by purification methods, freezing or Ca chelation.