Leachables such as formaldehyde, acetaldehyde, and N-3-(Dimethylamino)propyl)methacrylamide may leach into pharmaceuticals when in contact with polymeric equipment. There are indications that these leachables may interact with therapeutic proteins. Such effects were investigated by spiking human derived coagulation factor IX (FIX) with the leachables at concentrations of 1, 10, 50, 100, and 500 µg/mL. The spiked samples were visually inspected, and pH was measured. No visual effects were observed, and pH was within the drug product's specified range. Recovery analysis was performed to determine the unbound fraction of the leachables in the drug product solution. No loss of leachables was identified. Formaldehyde-induced binding to FIX was determined using protein structure analysis. Binding to lysine at two positions was demonstrated in a concentration-dependent manner, starting at 10 µg/mL. Finally, FIX clotting activity was measured. The clotting activity was decreased starting at 500 µg/mL formaldehyde and 50 µg/mL acetaldehyde, whereas N-3-(Dimethylamino)propyl)methacrylamide had no effect. Recovery - and protein structure analysis may provide information about interactions of leachables with FIX, but unlike clotting activity measurement, they do not necessarily conclude potential effects on biological activity. It is concluded that some leachables may indeed react with/or modify therapeutic proteins, potentially causing an insufficient pharmacological effect.