Development of faecal flora was studied in seven very low birth weight (VLBW) infants, who were fed on human milk and whose birth weights ranged from 810-1350 g. The intestine of the VLBW infants was first colonised by enterobacteria and streptococci, as it was in full-term infants. VLBW infants differed, however, from full-term infants in that both types of organism continued to be predominant for a longer period, and establishment of bifidobacterial flora was retarded. Bifidobacteria first appeared in the stools of VLBW infants at a mean age of 10.6 +/- 2.7 days and became predominant at a mean of 19.8 +/- 8.9 days, in contrast to full-term, breast-fed infants in whom bifidobacterial flora appeared at as early as 4 days of age. The delay seemed to be related to the low milk intake of the VLBW infants. The number of viable staphylococci in the stools of VLBW infants was generally higher than that in full-term infants. Although emergence of Bacteroides, Clostridium and lactobacilli was delayed compared with full-term infants, differences in their occurrence and prevalence between VLBW and full-term infants were not remarkable.