Escherichia coli strains isolated from newborn infants were injected intraperitoneally into infant rats. S possessing the K1 capsular polysaccharide antigen were significantly more virulent than strains lacking this antigen. When 5-day-old animals were injected with 1.2 x 101 colony forming units of a K1 E. coli strain (serotype O18ac:K1:H7), about 80% had bacteria isolated from their blood. Forty-eight percent of bacteremic animals had positive cerebrospinal fluid cultures. The development of bacteremia with >104 colony-forming units per ml of blood correlated with positive cultures of cerebrospinal fluid. Some animals, studied with serial blood cultures, were able to clear bacteria spontaneously from their blood, whereas others succumbed to infection within 48 h of challenge. The susceptibility of infant rats to E. coli infection was age dependent and appeared related to the K1 antigen.