A method has been developed to isolate measles virus proteins from infected hamster brain tissue. Suckling hamsters inoculated intracerebrally with the HBS strain of measles virus were used in these studies. Viral proteins were isolated from infected brain lysates by affinity chromatography on Sepharose beads coupled with IgG from rabbit hyperimmune anti-measles serum. The eluted proteins were separated by SDS-polyacrylamide gel electrophoresis (SDS-PAGE), electrophoretically transferred onto blotting matrix, and immunolabelled with anti-measles antibodies. Individual viral proteins were identified by labelling with monoclonal or monospecific antibodies. All viral proteins except the fusion (F 1) protein were identifiable on the immunoblots in relative amounts comparabe to purified virions. In addition, a second phosphoprotein (P) band not found in purified virions was present in infected brains and cell cultures infected with HBS or LEC strains of virus. This method should be useful for isolating small quantities of viral proteins from large amounts of tissue, and should make possible the characterization of measles virus proteins in persistently infected CNS tissue.