Protein A (from Staphylococcus aureus, Cowan I strain) is conjugated with fluorescein isothiocyanate (FITC) and is purified by molecular sieving to remove the unbound FITC and unconjugated Protein A. The product is in 10 mM phosphate buffered saline, pH 7.4, containing 1% ovalbumin, 40% glycerol, and 0.1% sodium azide.