Factor V was isolated from human plasma by barium citrate adsorption, polyethylene glycol fractionation, DEAE-Sepharose CL-6B chromatography, ammonium sulfate fractionation, and gel chromatography on Ultrogel 22. Degradation of Factor V during purification was largely prevented by ample use of inhibitors of proteolytic enzyme. The purified Factor V was a stable, single-chain molecule with an apparent molecular weight of 330,000. Activation of human Factor V by thrombin resulted in a 10- to 15-fold increase in activity. The activation pattern as monitored by sodium dodecyl sulfate polyacrylamide gel electrophoresis was compared with that of bovine Factor V. Differences in the patterns of thrombin activation were noticed between the two species, whereas the final products were similar. The products of human Factor V activation are two closely spaced doublets, one with an apparent molecular weight of approximately 110,000, and the other, approximately 72,000. An antibody was raised against the purified protein. Crossed immunoelectrophoresis showed that the antibody recognized Factor V both before and after activation with thrombin.