Abstract.The molecular‐weight distribution of proteins in factor VIII concentrates has been analysed by fast‐protein liquid chromatography. The proportion of high‐molecular‐weight (HMW) aggregates in one product increased on freeze‐drying and heating, with fibrinogen and fibronectin being the main protein components of the HMW peak. In all other concentrates, the HMW peak was less than or equal to 5% of the total protein content and there were no differences in HMW content according to purity or method of viral ina