Lyophilized or frozen plasmas with congenital deficiencies of factors II, VII, IX, or X, abnormal factor X (factor X Friuli) plasma, absorbed normal plasma, plasmas from patients treated with coumarin anticoagulants and normal plasma were studied in a crossover electrophoresis (electrosyneresis) system using an anti-factor X antiserum. A factor X band was present in plasmas with a deficiency of factors II, VII, and IX, in abnormal factor X plasma, and in normal plasma. No difference in the position of the factor X band was noted for all these plasmas. In coumarin plasma, two factor X bands were evident. The first of these two bands had the same position as the normal factor X band; the second one migrated slightly more towards the cathode. In adsorbed normal plasma and in factor X deficient plasma, no bands were seen. Minor, lighter bands were seen in most plasmas on the cathodic side of the slide and thought to represent secondary activities of the antiserum. These results suggest that the abnormal factor X (factor X Friuli) has the same crossover electrophoretic mobility as normal factor X, whereas the coumarin-induced abnormal factor X has a different mobility.