A total of 88 neonatal guinea pigs was inoculated intranasally with a clinical isolate of Escherichia coli K1 that had been passaged once in the peritoneum of an adult guinea pig; 74 animals became bacteremic, of which 54 had meningitis, 12 hr after inoculation. Bacteremia and/or meningitis occurred consistently in repeated experiments. It was possible to obtain serial specimens of cerebrospinal fluid by cisternal puncture without killing the animals. The induction of meningitis in a predictable fashion without disrupting the blood-brain barrier and the ability to study the cerebrospinal fluid without killing the animal make this a useful model of human neonatal meningitis caused by E. coli K1.