One of the primary goals of the hospital epidemiologist is the identification and characterization of nosocomial outbreaks. Outbreaks usually are identified by the recovery of a unique strain from a cluster of patients infected with a nosocomially acquired pathogen. Until recently, the microbiologic tools available to any hospital epidemiologist permitted identification of novel strains by speciation and antibiogram. Thus, most outbreak descriptions consisted of the identification of an unusual species or the appearance of a new antibiotic resistance phenotype in a recognized nosocomial pathogen. The emergence of enterococci in this decade or the emergence of methicillin-resistant Staphylococcus aureus (MRSA) in the 1970s am notable examples. Where resources have existed, additional methods have been used for further discrimination. Outbreak and endemic strains have been compared by phage typing, serologic typing, and capsular typing. For example, beta-hemolytic streptococci may be grouped by Lancefield antisera, and Staphylococcus aureus may be grouped by phage typing. However, certain Lancefield groups and certain phage groups are common among epidemic and endemic isolates, which may prevent identification of a specific epidemic strain.