20 patients with serious infections were treated with cefazolin and tobramycin for 9–64 days. Minimal inhibitory concentration (MIC) values and minimal bactericidal concentration (MBC) values were determined for each of the 112 clinical bacterial isolates. The median cefazolin MIC/MBC was 1.56/12.5 μg/ml, and the median tobramycin MIC/MBC was 6.25/25.0 μg/ml for all organisms studied. Median cefazolin doses of 49.0 mg/kg/day gave median peak/trough serum levels of 43.0/11.0 μg/ml. Median tobramycin doses of 4.3 mg/kg/day gave median peak/trough serum levels of 5.0/1.4 μg/ml. Checkerboard studies revealed synergy with 65% of strains. Bacteriologic and clinical success was obtained in 17 of 20 pat