Samples from the pharynx, stomach, duodenum or jejunum, and faeces were collected on 7 days between 1st and 28th day from neonates weighing less than 1.5 kg at birth who were fed by transpyloric tube. These were cultured on selective and non-selective media, and the results were expressed in a semi-quantitative manner. The number of bacterial species and the density of their growth increased with the patient's age; this was particularly noticeable with Gram-negative bacteria and the ratio of Gram-negative to Gram-positive organisms increased steadily in specimens from all sites with increasing age. The upper small intestine was more heavily colonised than the stomach early in life and the microflora present was predominantly faecal in nature. The species isolated from all sites were mainly aerobes or facultative anaerobes; strict anaerobes did not form a significant proportion of the microflora in these infants. Necrotising enterocolitis developed only after heavy jejunal colonisation with Gram-negative bacilli.