The antibacterial activity of cefotaxim and seven cephalosporins was determined in 1,112 fresh isolates using the microdilution technique. All of the cephalosporins tested were ineffective against enterococci. Cefalotin was the most effective agent against Staphylococcus aureus. Cefaclor was superior to cephalexin against Escherichia coli and Proteus mirabilis, and thus inhibited 20% more Enterobacteriaceae. Cefazolin was as effective as cefamandole against E. coli. Cefuroxime and cefoxitin were almost equally effective against E. coli, Klebsiella and P. mirabilis; more than 95% of the strains were sensitive. Cefuroxime and also cefamandole were much more effective than cefoxitin against Citrobacter and Enterobacter. Cefoxitin on the other hand was superior against Serratia and indol-positive Proteus species. Cefotaxim was by far the most active cephalosporin against Enterobacteriaceae, only 2% of the strains being resistant. More than 90% of the strains of E. coli, Klebsiella, P. mirabilis and Serratia were sensitive to 1 mg/l, the lowest degree of activity being displayed against Enterobacter; 82% of the strains were inhibited by 16 mg/l. Cefotaxim was the only cephalosporin which showed appreciable activity against Pseudomonas.