There has been no systematic re-examination of variables that may affect the level and duration of response of patients with hemophilia B (Christmas disease) to transfusion. Therefore, 49 of our transfusion episodes and 171 previously reported transfusions were evaluated. Mean calculated initial increase of Factor IX levels (delta %/unit (U) of procoagulant activity infused/kg) was 0.82 +/- 0.09% (mean +/-S.E.) in previously reported cases and 1.01 +/- 0.13% in our patients, after transfusion of concentrate; but only 0.05 +/- 0.11% after fresh frozen plasma (FFP). Response was not altered by acute hemorrhage, baseline Factor IX levels, or body weight. Proplex (Hyland) and Konyne (Cutter) produced similar responses. Following transfusion, the disappearance curve was biphasic. The mean T1/2 for the second component was 27.5 hrs, but the direct T1/2 was only 6.4 +/- 1.0 hr. Regardless of common clinical variables, increase of Factor IX following transfusion of American concentrates is 1.0% (or 0.01 U)/1 administered/kg. Appropriate frequency of transfusion depends upon an understanding of the biphasic disappearance of Factor IX. Importantly, the initial frequency of transfusion therapy should be based on a direct T1/2 of only 6 to 8 hrs.