A randomised double trial was carried out in 266 women who had Caesarean without any high risk of infection in order to study the efficacy of prophylactic antibiotics given during the operation. One group received 1 gram of cefotetan when the cord was being clamped and the other had an injection of placebo under the self-same conditions. Apart from studying the clinical efficacy, evaluation of the economics of the treatment was carried out using, as parameters, the length of stay in hospital and the cost of the antibiotics which were prescribed after the operation. The following results were obtained: 75% of the Caesarean operations carried out in the Maternity Units of the University Regional Hospital Centre were without high risk of infection. Prophylactic antibiotics are proficient because they reduce post Caesarean morbidity due to: endometritis, superficial and deep abscesses and septicaemia. 12.5% in the group who had antibiotics developed infections as against 26% in the placebo group. Post Caesarean infections which required antibiotics cost on an average for each Caesarean 16 francs in the groups who received antibiotics as against 52 francs in the groups that received the placebo. Even including the cost of the antibiotics given prophylactically the costs of antibiotics (prophylactic and curative) was higher in the antibiotic group than in the placebo group. The length of hospital stay was significantly reduced in the group that received prophylactic antibiotics.