Bacteriologic study of amniotic fluid was undertaken in 300 cases of prolonged rupture of the membranes to evaluate the risk of infection without systematic administration of prophylactic antibiotics to the mother. The rate of amniotic contamination was 8.3 per cent within the first 12 hours following rupture of the membranes, it increased with time to reach 52 per cent, after 48 hours. In 90 per cent of the cases, contamination was caused by streptococci and anaerobes, Streptococcus B alone being responsible for 50% of the contaminations. Antibiotics administered to the mother were effective, but they needed a 6 hour-minimum delay to sterilize a contaminated amniotic fluid. Although the overall percentage of contamination was 23 per cent, infection occurred in only 4 per cent of the infants. The outcome was satisfactory in every case. Those results do not seem to discredit the suppression of prophylactic cover antibiotic therapy following the premature rupture of the membranes.