In January and February of 1981, 85 cases of enteritis caused by Salmonella muenchen were reported from Ohio, Michigan, Georgia, and Alabama. Initial investigation failed to implicate a food source as a common vehicle, but in Michigan 76 per cent of the patients, in contrast to 21 per cent of the control subjects, admitted personal or household exposure to marijuana (P less than 0.001, relative risk = 20). Marijuana samples obtained from patients' households contained as many as 10(7) S. muenchen per gram. The outbreak-related isolates of S. muenchen were sensitive to all antibiotics and were phenotypically indistinguishable from other S. muenchen. Plasmid fingerprinting, however, revealed that all isolates related to marijuana exposure contained two low-molecular-weight plasmids (3.1 and 7.4 megadaltons), which were absent in control strains. Plasmid analysis of the isolates showed that the outbreaks in Ohio, Michigan, Georgia, and Alabama were related, and analysis of isolates submitted from various other states demonstrated that cases associated with marijuana may have been dispersed as far as California and Massachusetts.