Polymorphonuclear leukocyte (PMN)-resistant strains of Pseudomonas aeruginosa had the ability to interfere with phagocytosis of Escherichia coli, whereas PMN-sensitive strains of P. aeruginosa did not. When mice were infected with an ordinarily nonpathogenic strain of E. coli, addition of a PMN-resistant strain of P. aeruginosa gave a mortality considerably higher than that obtained with P. aeruginosa alone, whereas addition of a PMN-sensitive strain of P. aeruginosa gave a mortality not different from that observed with P. aeruginosa alone. This increased mortality in mixed infection with E. coli and PMN-resistant P. aeruginosa was obtained by facilitation of tissue invasion by both bacteria from the inoculum site.