A specific labeling procedure for proteins on the outer surface of membranes.

The proteins located on the exterior of the membrane of influenza virus can be preferentially labeled by incorporation of tritium. This incorporation is achieved by sodium borotritide reduction of the Schiff’s base formed between pyridoxal phosphate and protein amino groups of purified virions. The preferential labeling is due to the inability of phosphate esters, such as pyridoxal phosphate, to penetrate cell membranes. Labeled virions remain fully infectious and retain hemagglutinating activity.