The immediate effect of parenteral antibiotic treatment (gentamicin and ampicillin) on the aerobic and anaerobic faecal flora of 14 infants with suspected or proven neonatal septicaemia was studied. Eight infants of similar gestational and postnatal age were studied for comparison. All control infants showed an abundant growth of both aerobic and anaerobic bacteria as early as the first 2 weeks of life. The treated infants generally had lower counts of aerobic and especially of anaerobic bacteria; in 10 of 16 cultures no anaerobes were isolated. In relation to aerobic strains E. coli dominated in untreated infants and Klebsiella pneumoniae in treated ones.