The relatively low yield of factor II activity in prothrombin complex preparations gave rise to an investigation in which factor II was determined on the basis of its coagulation activity and on that of its antigenic properties during the routine preparation procedure of the complex. It appeared that factor II, measured as activity, suffered a greater loss than factor II, measured as antigen, during this procedure. This greater loss had to be attributed to changes in the conformation of the factor II protein molecule induced by the freezing step of the preparation method hitherto in use. Omission of this step led to a higher yield of factor II activity in the prothrombin complex preparation. Therefore, freezing and thawing during the preparation of the prothrombin complex should be avoided.