A method is described for isolating and characterizing external lymphocyte surface proteins. Intact 125I-labelled tonsillar lymphocytes were incubated with antilymphocyte serum, solubilised with NP-40 precipitated with Staphylococcus aureus and the labelled proteins analysed on SDS polyacrylamide gels. The molecular weights of the proteins labelled in this way were compared with those of surface antigens labelled by the galactose oxidase method. This method may also be used for isolation of surface receptor molecules which lose their stereochemical structure upon solubilisation of the cells.