K88 antigen, and important virulence factor in porcine enteropathogenic Escherichia coli (EEC), can be transferred along with the ability to ferment the trisaccharide raffinose (Raf). The plasmids from a number of EEC strains that encode these two properties were isolated and characterized. In most strains the K88 and Raf genes were found on a single nonconjugative plasmid approximately 50 x 10(6) daltons in size. This plasmid core was conserved with only slight variation among the strains tested. In some transconjugants, larger conjugative plasmids were observed that were apparently recombinants between the Raf/K88 plasmid and a transfer fa(tor. Occasionally plasmids carrying only the raffinose fermentation genes arose by deletion of a deoxyribonucleic acid segment of about 20 x 10(6) daltons that included the K88 antigen gene(s).