Lymphocytic choriomeningitis (LCM) has been rarely reported in the American literature since 1960. It is interesting that each of the 3 epidemics reported since then has been associated with exposure to hamsters. In 1973, 48 cases of LCM spanning the years 1971-1973 occurred at the University of Rochester Medical School associated with hamsters implanted with tumour tissues. These tissues were found to be LCM-positive, as in an earlier outbreak in 1965 at the National Institutes of Health. A nationwide outbreak of LCM occurred in late 1973 and early 1974 totalling at least 181 cases in 12 states; all were associated with pet hamsters from a single breeder in Birmingham, Alabama. He was an employee of a biological products firm whose tumour tissues were found positive for LCM and were also incriminated in the 1973 Rochester outbreak. The last outbreak occurred in a graduate school laboratory in New York State involving 7 individuals working with hamster tumours from the same Birmingham biological firm. The nationwide epidemic ended in middle April 1974 following removal of incriminated hamsters from pet shops throughout the country and voluntary cessation of distribution of hamsters from the incriminated breeder. The biological firm notified all laboratories of the possible contamination of tumours and has voluntarily stopped distribution of known positive tumours.