Among 147 gram-negative uropathogens, 49 per cent are susceptible to cefazolin, 39 per cent to cephapirin, 32 per cent to cephaloridine, 31 per cent to cephalothin, 19 per cent to cephalexin, 15 per cent to cephadrine, and 0.07 per cent to cephaloglycin. Two thirds ofEscherichia coliare susceptible to cefazolin, 46.5 per cent to cephapirin, and 37 per cent to cephaloridine. Sixty-four per cent ofKlebsiella pneumoniaeare susceptible to cefazolin, 54.5 per cent to cephalexin, and 50 per cent to cephapirin. Half of the indolepositive and 71 per cent of indole-negativeProteusare susceptible to cephalothin. As there is a wide variation among pathogenic bacteria to seven tetracyclines, a similar variation exists among seven cephalosporins. On the basis of these data, it is advisable to test a pathogen against all seven cephalosporins.