A nosocomial outbreak of Salmonella enteritidis phage type 4 occurred in July 1995. Seven definite cases were identified over 13 days affecting four wards in a London hospital. The outbreak strain was characterized by plasmid profile typing and pulsed-field gel electrophoresis (PFGE), and was unusual in that it did not possess a 38 MDa plasmid common to most isolates of S. enteritidis PT 4 made from humans and food animals in England and Wales. Seven asymptomatic excreters were identified on screening. No additional cases occurred on wards after standard isolation procedures were implemented. No common or continuing food or dietary source was identified. Results of epidemiological, microbiological and environmental investigations suggested that the outbreak was due to person-to-person transmission within the hospital. The source of the outbreak was not established but was probably due to admission of a patient with an unrecognized infection of S. enteritidis PT 4. The study highlights the importance of close collaboration between hospital staff, epidemiologists and microbiologists, and demonstrates the value of molecular techniques for strain subdivision in outbreak investigations.