Purified human factor VIII was incubated for up to 24 hours with plasmin, and the activity of the breakdown products studied at intervals. Factor VIII coagulant activity was lost within the first hour, but von Willebrand factor activity (FVIIIR:WF) was retained for two hours, and then declined slowly during the subsequent incubation. Analysis of the 24-hour breakdown products by immuno-electrophoresis, sepharose 4B chromatography and SDS Polyacrylamide electrophoresis revealed three main groups of fragments recognised by rabbit anti-human factor VIII anti-serum, and having molecular weights in the following ranges: Group 1 300,000=500,000; Group II, 150–200,000; Group III, 100,000. FVIIIR:WF activity, which was found only in Group II, appeared to be associated with glycopeptide(s) of up to 155,000 daltons.