Mature human factor IX is a 55,000-d glycoprotein with a modular domain structure and numerous posttranslational modifications. A recombinant form of human factor IX (rFIX) has been produced from a Chinese hamster ovary cell line that was engineered for high-level protein processing and expression. To ensure that the recombinant molecule contains the requisite structural and functional features of the plasma-derived form, rFIX was subjected to detailed biochemical and biophysical characterization. The laboratory studies showed that the posttranslational modifications and primary, secondary, and tertiary structures of rFIX were similar to those of plasma-derived factor IX (pdFIX). In addition, rFIX displayed a high degree of purity and a product release specification for specific activity that is > or = 200 IU/mg.