For quantitative determination of human Factor IXa (FIXa) activity as contamination in human Factor IX (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. FIXa activity in FIX containing concentrates is determined in a chromogenic method, in which human FX is activated by contaminating human FIXa in the FIX concentrate in the presence of FVIII, thrombin, calcium ions and phospholipid. The amount of generated human FXa is determined from the hydrolysis of a chromogenic FXa substrate. The sample FIXa activity is determined by the slope ratio model in which the potency of the sample is calculated vs. a FIXa standard with potency expressed in International Units (IU).