The size of human factor VIII-related protein in plasma has been determined by sodium dodecyl sulfate (SDS) glyoxyl agarose electrophoresis. The protein was immobilized after the electrophoresis by coupling it to the modified agarose, and it was identified by autoradiography using purified rabbit anti-factor VIII-related antigen (VIIR:Ag). A series of multimeric forms was identified with Mr of 0.85- 12 x 10(6). The distribution of VIIR:Ag multimers was the same in heparin and citrate anticoagulated plasmas and in serum, and the pattern was the same after freezing as in plasma kept at 37 degrees C from the time of venipuncture until the electrophoresis was complete. These observations indicate that VIIR:Ag circulates in normal plasma as a population of very large multimers and that the size distribution is not an artifact induced by purification methods, freezing, or calcium chelation.