For determination of Factor IX (FIX) activity in plasma and FIX containing concentrates. FIX is a single chain, vitamin K dependent glycoprotein of about 55 KDa, which is activated by FXIa or tissue factor / FVIIa. Activated FIX (FIXa) converts FX to FXa in the presence of FVIII, phospholipids and calcium ions. FIX activity is determined in a chromogenic method, in which human FIX is activated by human FXIa and where formed FIXa activates human FX in the presence of human FVIII, calcium ions and phospholipid. Similar to in vivo conditions, FVIII is activated by thrombin which is generated during the incubation. The amount of FXa formed is related to the FIX activity and is determined from the hydrolysis of a chromogenic FXa substrate. The FIX activity of the sample is assigned vs. a FIX plasma or a FIX concentrate standard with FIX potency expressed in International Units (IU).