Chemical treatment of human factor VIII by sodium periodate resulted in increased levels of procoagulant activity when measured by the two-stage assay. Increased levels of factor-VIII activity were observed following periodate treatment of factor-VII concentrates and normal, haemophilic and von Willebrand's disease plasmas. However, sodium periodate treatment of haemophilic plasmas which were completely deficient in factor-VIII activity did not cause any reduction in the clotting times of two-stage assays. No increase in procoagulant activity following oxidation of factor VIII could be detected by the one-stage assay method. Gel filtration of factor-VIII concentrates before periodate treatment showed that the protein exhibiting factor-VIII activity had an apparent molecular weight in excess of 1 000 000, whilst after periodate treatment the apparent molecular weight was reduced to 690 000.