Donor foal serum is the liquid fraction of whole blood that is collected after the blood is allowed to clot; it is the blood plasma with the fibrinogens removed. Serum includes all proteins not used in blood clotting (coagulation) and all the electrolytes, antibodies, antigens, hormones, and any exogenous substances. Donor foal serum is used as a blocking agent in immunoassays and controls.