A method is presented for the efficient conjugation of horseradish peroxidase to alpha-bungarotoxin. The 1:1 molar conjugate obtained is purified to completion by gel filtration on Sephadex G-100, followed by ion exchange chromatography on CM-Sephadex. The conjugate retains half of the activity of unmodified horseradish peroxidase and binds effectively to the nicotinic acetylcholine receptor of muscle. The conjugate is proven to be useful reagent for the histochemical staining of the receptor on muscle fibers for light and electron microscopy.