Fifty-one babies with prolonged rupture of fetal membranes (longer than 24 hours) were studied for evidence of latent infection. Cord blood was taken from all babies for a full blood count and blood culture. Gastric aspirates were collected and vernix swabs were taken immediately after delivery. Microscopy, culture and antibiotic sensitivity tests were done on the appropriate specimens. Each baby had a thorough medical examination immediately after birth, and 3 and 7 days afterwards. After the initial investigation they were allocated to a treatment (penicillin 50 000 U/kg/d and kanamycin 10 mg/kg/d) or a non-treatment group by randomised card selection. Six patients became infected, as was shown by a positive blood culture, while 4 of the 6 had clinical signs of infection as well. These infants were treated with penicillin and kanamycin, and all did well. None of the remaining babies showed any signs of infection. There were no deaths. Blood culture was found to correlate well with clinical infections, and many be used as a guide to latent infection and treatment.