In preparing a factor IX concentrate with a high yield and low hepatitis and thromboembolic risks, we have tested this material for survival in an in vivo system, the hemophiliac dog. By following the disappearance of radiolabeled, isolated factor IX in addition to the classic clotting assays, data on protein survival and more accurate kinetic parameters were obtained.Crude factor IX concentrate was prepared by batchwise adsorption-elution with DEAE-Sephadex using cryoprecipitate-poor human plasma. Isolated human factor IX was radiolabeled with 125I by chloramine-T without in vitro loss of clotting activity (Thompson, J Clin Invest, in press, 1977). A preparation containing both crude and isolated factor IX was then subjected to filtration (0.22 μm) and lyophilization; clotting and radioactivity were not altered by these steps.Following infusion of the combined preparation into a dog with severe hemophilia B (0% baseline factor IX) 10 post infusion samples were taken over 96 h for determination of radioactivity and factor IX clotting activity. These data were then analyzed by fitting to a two exponential expression using a Marquart non-linear least squares numerical procedure for a two compartment open model. The central volume was 14.5% of the animal’s body weight; the total volume of distribution was 28% with a t 1/2 distribution of 114 min. The t 1/2 elimination was 20 h; the slower phase of elimination (β, or that affected by redistribution) had a t 1/2 of 40 h. Factor IX clotting activity from the crude concentrate closely paralleled radioactivity from the isolated factor IX throughout the 96 h; t 1/2 β was slightly longer from the clotting activity data.