Human neonates are uniquely susceptible to serious infections due to Escherichia coli. Investigation of the serotypes of E. coli isolated from neonates with meningitis revealed that greater than 80% of the isolates possessed the capsular polysaccharide antigen designated K1. Cultures of stool from healthy infants, children, and adults have shown that K1 organisms are commonly found in individuals of all ages. Studies of mother-infant pairs have demonstrated transmission of K1 strains from mother to infant shortly after birth. In the rare infant who develops meningitis due to E. coli K1, serotype analysis frequently reveals that the same organism was isolated from the infant's cerebrospinal fluid and from the stools of both mother and infant. Preliminary investigations of humoral immunity demonstrated an age-related acquisition of antibodies to the capsular polysaccharide of E. coli K1 in healthy infants and children. An animal model was developed in which feeding of E. coli K1 to infant rats resulted in colonization, bacteremia, and meningitis in the animals.