The purpose of this study was to determine the prevalence and vertical transmission of anaerobic infection in early onset neonatal septicaemia in babies born to mothers with adverse perinatal factors. One-hundred neonates born to mothers with one or more adverse perinatal factors were prospectively studied. Aerobic and anaerobic blood cultures were drawn in addition to rapid diagnostic tests. High vaginal swabs were also taken from all mothers prior to delivery. Anaerobic Clostridium species infection was found in 5 per cent babies and all of them were symptomatic. Fifteen babies had aerobic infection. There was one baby with concomitant aerobic and anaerobic infection. There was no significant difference in sex distribution, birth weight, or gestational age between babies with aerobic and anaerobic infection. Similarly, there were no symptoms unique to anaerobic infection. None of the rapid diagnostic tests had a good sensitivity for anaerobic infection. Fifty-two per cent of the mothers had anaerobic growth in their vaginal swabs, but none had transmitted to their babies. All five babies recovered within 24-72 h of commencing therapy. This study shows that anaerobes have a role in early onset neonatal septicaemia. They cause serious morbidity, but carry a good prognosis. For identification of such infection blood culture is mandatory.