Proteins of a number of measles and SSPE virus strains have been compared by limited proteolysis and they appear to be largely conserved amongst the various strains. Viruses derived from SSPE cannot be distinguished from other measles viruses by this technique. Small differences in the digest patterns of the M proteins have been observed between the Edmonston and other measles virus strains. Furthermore, in some strains where the M proteins migrate slower in SDS-PAGE the limited proteolysis patterns are slightly different from those in other MV and SSPE virus strains. The limited proteolysis pattern of some canine distemper virus (CDV) proteins have been determined and nucleocapsid breakdown products have been identified in infected cells. Comparisons of proteins of four strains of CDV have shown that these, too, are largely conserved, although the digest of proteins of CDV appear to show more pronounced differences than those present in the MV and SSPE virus group. Limited proteolysis can be used readily to distinguish MV from CDV isolates.