Fifty-nine Escherichia coli strains belonging to two clonal groupings were investigated for major outer membrane proteins, colicin production, and partly for plasmid DNA content. The membrane protein patterns of the 01:K1:H7(H-):F11 and O1:K1:H-:F9 strains obtained by sodium dodecyl sulfate-polyacrylamide gel electrophoresis were distinctly different from each other and, therefore, are useful for clonal assignment. All of the F11 isolates had one plasmid of about 85 Md in common which is suggested to be characteristic for the clone. Their content of smaller plasmid DNA was heterogeneous and showed geographical differences. The F9 strains showed a plasmid pattern different from the F11 strains. A plasmid of 2.6 Md was found in all of these isolates. Colicin production was found to be useful for clonal assignment only in two thirds of the F9 and not in the F11 strains. Some strains with identical properties seem to be of epidemic importance.