Escherichia coli K1 strains, isolated from human newborns with meningitis, were fed to pathogen-free Sprague-Dawley infant rats by an oral gastric tube. Feeding of 10(3) to 10(11) organisms colonized the intestine of approximately 70% of the animals. At 5 days postfeeding of 3- to 5-day-old rats, bacteremia was detected in 60%, and meningitis occurred in 15% of bacteremic animals. Colonization and bacteremia were age-related. Rats 15 days old had only 19 colonization and 10% bacteremia, and those 30 days old were almost completely resistant to colonization and bacteremia. The intranasal route was less effective in inducing colonization and bacteremia. Intralitter transmission from E. coli K1-fed rats occurred, with 52% of water-fed controls becoming colonized and 15% become bacteremic. Colonization of mothers from their fed infants occurred, but none of five tested developed bacteremia. Other E. coli capsular polysaccharide types were studied. A K92 strain isolated from a newborn with meningitis induced a 77% colonization rate, and 8% of these developed bacteremia without detectable meningitis. An E. coli K100 strain showed a 32% colonization rate, and 2% developed bacteremia. The age relation, relatively high virulence of K1 compared with other capsular types, spontaneous appearance of colonization, bacteremia, and meningitis, and intralitter transmission of colonization and disease in newborn rats closely parellel E. coli epidemiology in human neonates.