A virulent isolate of Pseudomonas aeruginosa PAO1, which had been obtained from eight sequential intraperitoneal infections in mice compromised with iron and methotrexate, expressed greater lethality than the avirulent parent strain when both strains were injected into mice treated with iron. The present study demonstrates that pyochelin, a siderophore produced by P. aeruginosa, also increases the lethality of the virulent bacteria but not of the avirulent bacteria. Analysis of the growth and clearance of both virulent and avirulent strains in mice revealed that pyochelin increased the growth and lethality of virulent bacteria but only increased the survival of the avirulent bacteria. A streptomycin-dependent mutant of strain PAO1 (strd1) was used to demonstrate that pyochelin did not affect the clearance activity of mice. This strongly suggests that the effects of pyochelin in stimulating the persistence of avirulent bacteria and in increasing the lethality of virulent bacteria are due solely to the promotion of bacterial growth. Since the virulent bacteria were equivalent to the avirulent bacteria in utilizing pyochelin during in vitro growth in the presence of transferrin, it appears that the stimulation of growth by pyochelin allows the expression of additional virulence properties by the virulent bacteria.