In vitro antibacterial activities of 9 antibiotics including aztreonam (AZT) against clinically isolated Gram-negative bacteria were determined using MIC-2000 plus system. Bacteria were isolated from clinical materials in Saga Medical School during a period from May 1987 to March 1988. Summarized results were as follows: 1. AZT showed excellent antibacterial activities against Escherichia coli, Klebsiella pneumoniae, Proteus sp. and Haemophilus influenzae, and MIC80 values of AZT against these organisms were lower than 0.20 microgram/ml. 2. Antibacterial activities of AZT were superior to cephem antibiotics compared against Enterobacter aerogenes, Enterobacter cloacae, Citrobacter freundii and Serratia marcescens. 3. The MIC50 and MIC80 of AZT against Pseudomonas aeruginosa were 12.5 micrograms/ml and 25 micrograms/ml, respectively. 4. AZT did not show any antibacterial activity against Acinetobacter sp. and Xanthomonas maltophilia.