To investigate the bacterial response to antibiotic stress, we analyzed the outer membrane proteins of Pseudomonas aeruginosa grown in the presence of a sub-minimum inhibitory concentration of antibiotics. Among the antibiotics tested, fluoroquinolones and streptonigrin induced a large amount of outer membrane protein with a molecular mass of 43 kDa. This protein is most likely the stress-responsive protein, since the quinolone-resistant mutants with a higher minimum inhibitory concentration of antibiotic than the wild-type strain produced a large amount of 43-kDa protein only in the presence of sub-minimum inhibitory concentration of the mutants itself, but not that of the antibiotic-susceptible wild-type strain. The sequence of N-terminal 15 amino acids of the 43-kDa protein was identical to that of pyocin R1. However, purified pyocin R1 failed to accumulate in the outer membrane. Thus, we concluded that the 43-kDa protein (pyocin R1) is the antibiotic-stress-induced outer membrane protein.