The plasmid mediation and transmissibility of heat-stable enterotoxin production and multiple antibiotic resistance have been demonstrated for Escherichia coli O78:K80:H12 epidemiologically incriminated in a hospital outbreak of infantile diarrhea. The conjugal transfer of a 67 X 10(6) - and a 30 X 10(6)-dalton plasmid was associated with the transfer of resistances and enterotoxin production, respectively. Using antibiotics to select E. coli K-12 transconjugants from a one-step bacterial cross, all of the monitored resistances were transferred concurrently, and 36% of the resistant transconjugants produced enterotoxin.