An outbreak of lymphocytic choriomeningitis (LCM) virus infections occurred in Medical Center personnel at the University of Rochester in 1972-1973. A total of 48 infections was discovered, 31 in staff of the radiation therapy area, 12 in the staff of the Vivarium facility, and 5 in other individuals who worked in the Medical Center. Twenty-one of the infections were associated with recent febrile illness with myalgia. Epidemiologic and virologic studies indicated that the source of the infection was Syrian hamsters which were used in tumor research; the tumor cell lines themselves were shown to be contaminated with LCM virus and probably represented the original source of contamination of the Rochester facility. Infection occurred not only through direct contact with infected animals but also from mere presence in the room where the animals were held. This outbreak emphasizes the need for restricting access to animal quarters and for surveillance of hamsters and tumor cell lines and indicate the necessity of separating animal facilities from patient-care facilities.