The full-length nucleic acid sequence of the C.sub.1 bacteriophage is disclosed in the present application. The specific regions of the C.sub.1 genome encoding the PlyC lysin have also been identified and sequenced. The invention relates to the pharmaceutical and diagnostic utility of these sequences and provides for development of pharmaceutical compositions for treating or preventing streptococcal infections in mammals, for compositions for decontamination of inanimate surfaces and for diagnosis of streptococcal infections.