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Escherichia coli F-18, isolated from the feces of a healthy human, is an excellent colonizer of the large intestines of streptomycin-treated CD-1 mice. E. coli F-18 Col-, a poor mouse colonizer relative to F-18, lacks a 3 X 10(7)-dalton plasmid present in E. coli F-18. Both strains are human type A erythrocyte hemagglutination negative, have identical surface hydrophobicities, contain the same number of lipopolysaccharide molecules with the same O-side chain length, and have identical amounts of capsule. Differences between the two strains were observed. The relative amounts of specific outer membrane proteins differed, and E. coli F-18 was less motile than E. coli F-18 Col-. The abilities of the two strains to bind mouse large intestine mucous gel was also examined. Although each strain was able to use mucous gel as the sole source of carbon and nitrogen with equal ability, E. coli F-18 bound between two and three times more mucous gel than did E. coli F-18 Col-. Most of the difference in mucous gel binding ability of the two strains was accounted for by the greater ability of E. coli F-18 lipopolysaccharide to bind a single 26,000-dalton mucous gel protein. E. coli J5-3, a typical K-12 strain that is also a poor colonizer relative to E. coli F-18, was identical to E. coli F-18 Col- with respect to mucous gel binding ability.