The in-vitro activity of ceftazidime, ceftriazone, cefoperazone and cefotaxime was studied in 900 clinical isolates. No significant difference was found in antibacterial activity between the Chinese-made and the imported ceftazidimes. Ceftazidime was highly active against streptococci but less active against staphylococci and inactive against methicillin-resistant S. aureus and enterococci. Ceftazidime showed strong activity against Enterobacteri aceae except E. cloacae. 8 µg/ml of ceftazidime inhibited 96.6% strains of Enterobacteriaceae, 97% of Ps. aeruginosa, 90% of acineto-bacter lwoffi and 65% of A. anitratus. With H. influenzae and N. meningitidis, the MICs were ≤0.06 - 0.125 µg/ ml. All the cephalosporins tested were poorly active agsinst Bacteroides spp. The in-vitro bacterioidal effects against E. coli and Ps. aeruginosa were demonstrated by killing curves. Our data suggested that ceftazidime could be an useful antibiotic in the treatment of severe infections caused by Gram-negative organisms, especially the Ps. aeruginosa.