A method of preparing an enzyme-labeled ligand for use in specific binding assays for determining said ligand or a specific binding partner thereof, by covalently coupling said ligand or a binding analog thereof to an organic prosthetic group and combining the resulting coupled product with an apoenzyme to form a conjugated enzyme complex with said prosthetic group in said coupled product. Preferably, the conjugated enzyme complex in the resulting labeled conjugate exhibits holoenzyme activity which is affected by binding of said labeled conjugate by a specific binding partner of said ligand. Preferred prosthetic groups are flavin mononucleotide, heme, and, especially, flavin adenine dinucleotide, with preferred apoenzymes being apoperoxidase and, especially, apoglucose oxidase.