Human Factor VIII procoagulant protein (VIII:C) is a plasma protein that participates in the cascade of events leading to blood coagulation. It is absent or defective in patients with hemophilia A. In vivo Factor VIII:C associates with Von Willibrand factor and its multimers to form a high-molecular-weight particle that can be dissociated into a lower-molecular-weight form in the presence of high concentrations of salt. We have been able to purify rapidly Factor VIII:C on a large scale by sequential high-performance size-exclusion chromatography (HPSEC) under conditions of first low salt and then high salt concentration. Reconstituted commercial Factor VIII:C concentrate was purified by chromatography on a preparative HPSEC column (Toyo Soda, 60 X 2.5 cm, 300 ml) in 0.05 M imidazole buffer, (pH 7.0), containing 0.15 M sodium chloride. Factor VIII:C activity was eluted in the void volume in less than 20 min as a high-molecular-weight particle, well separated from low-molecular-weight contaminants. Purification was 20-fold, with a yield of 80%. Up to 4 g of Factor VIII concentrate could be purified at one time in this manner. This material was then concentrated and made 0.35 M in calcium chloride prior to re-chromatography on the same column in a buffer containing 0.30 M calcium chloride. Under these conditions, Factor VIII:C activity was eluted in the inner volume of the column at a position corresponding to a molecular weight of several hundred thousand in less than 1 h. It was well separated from both larger proteins and smaller peptide fragments.(ABSTRACT TRUNCATED AT 250 WORDS)