I have examined 20 plasmids conferring chloramphenicol resistance (Cm) in multiresistant strains of Escherichia coli pathogenic for man and piglets. In Denmark, one plasmid family, exemplar pHG33, is responsible for all chloramphenicol resistance in serotypes of E. coli found in diseased piglets. A closely related plasmid, pHG50, was identified in an enteropathogenic E. coli (EPEC) strain from an infant. The isolate was epidemiologically unrelated to the piglet isolates. The molecular relatedness of the plasmids was established by restriction enzyme analyses and Southern blots. Chloramphenicol resistance plasmids in E. coli from urinary tract infections, or in English EPEC strains, did not show the same close relatedness with the piglet plasmid pHG33. However, many were of the same incompatibility group and their restriction profiles displayed a number of common bands. The close relatedness of pHG50 and pHG33 suggests exchange of plasmids between pathogenic serotypes of E. coli from man and animals. The infant from whom the EPEC strain carrying plasmid pHG50 was isolated might have acquired it from piglets. Disease in human babies caused by EPEC strains is now rare in Denmark and no Cm-resistant strains are found. Possible reasons for the loss of Cm-resistance plasmids from human strains and their retention in piglet strains are discussed.