Antibodies to capsular polysaccharides of group C Neisseria meningitidis are often found in sera of young adults despite infrequent nasopharyngeal carriage and low rate of attack of N. meningitidis in the United States. Thus, experiments were designed for detection of bacteria cross-reactive with N. meningitidis. Among 3,264 cultures of stool, urine, blood, and cerebrospinal fluid, only 14 strains were found to be cross-reactive; all were Escherichia coli possessing the K92 capsular polysaccharide. The somatic O-antigens were 16, 13, 23, and 73; the flagellar antigens were H4 and 34. All K92 strains of E. coli showed the expected fermentations, were sensitive to common antibiotics, and lacked enteropathogenicity. Antigens of both E. coli K92 and group C N. meningitidis are capsular, acidic polysaccharides composed of sialic acid. The K92 polysaccharide is N- but not O-acetylated, sensitive to neuraminidase, and linked by alpha-2,8- alternating with alpha-2,9-ketosidic bonds. The K92 polysaccharides from all E. coli studied had similar biophysical and immunological properties. Intravenous injection of formalin-treated K92 organisms induced precipitating and bactericidal antibodies to polysaccharides of N. meningitidis. E. coli K92 strains may provide an alternative immunogen for prophylaxis against disease due to group C N. meningitidis in infants and young children.