SummaryThe in vitro antibacterial activity of cefoxitin against 150 cephalothin resistant Enterobacteriaceae (MIC≥16μg/ml) was compared with that of cefamandole, cefuroxime, cefazaflur and cefaclor, by using the Kirby Bauer disc method. Seventy-five percent of the cephalothin resistant strains were still sensitive to cefoxitin, except forEnterobacterandCitrobacterstrains, cefoxitin was also the most active drug against each group of Enterobacteriaceae with susceptibility ranging separately from 86 % to 100.Enterobacteriaceae isolates, representing 55 % of the total number, play still an important role in hospital pathology, although the increase in the number of isolates was less than that of non-Enterobacteriaceae.Finally, 30 % of our Enterobacteriaceae, isolated in September and October 1978, were resistant to cephalothin, although once again there seems to be an improved sensitivity to cephalotin from 64 % to 70 % in 1978.Because of these in vitro studies the introduction of a new (3-Iactam antibiotic in the hospital formularium seems justified.