Fifty clinical isolates of Pseudomonas aeruginosa were tested for inhibition of growth of clinical isolates of Escherichia coli, Salmonella infantis, Klebsiella pneumoniae and other Gram-negative bacteria in the authors' laboratory. Pseudomonas aeruginosa was strongly active against both E. coli and Enterobacter cloacae, with 89.4% and 94.7% inhibition respectively, but weakly active against S. infantis, K. pneumoniae and Proteus mirabilis with 56.3%, 48.8% and 23.8% inhibition, respectively. The pigmented strains were found to have stronger antimicrobial activity than the unpigmented strains. Pyocyanin, the major metabolite of Pseudomonas aeruginosa, has been shown to inhibit Escherichia coli, Proteus spp. and other Gram-negative bacteria, by research with a few strains of P. aeruginosa and a single inhibited strain. However, little attempt has been made to determine the inhibitory action of many strains of P. aeruginosa against a large number of clinical isolates such as Escherichia spp., Klebsiella spp., and Salmonella spp., up to now. For this reason, in this study we examined 50 randomly selected clinical isolates of P. aeruginosa for inhibition of growth of a wide range of Gram-negative bacteria, including 30 strains of E. coli, 30 of K. pneumoniae, 30 of S. infantis, 6 of Enterobacter cloacae and 9 of Proteus mirabilis.