The effects of the Pseudomonas aeruginosa-derived pigments, pyocyanin and 1-hydroxyphenazine (1-hp), on membrane-associated oxidative metabolism and release of lysozomal enzymes by human neutrophils were investigated in vitro. Pyocyanin, but not 1-hp, increased the generation of superoxide and the rate and duration of oxygen uptake by activated neutrophils. Both agents increased the myeloperoxidase-mediated iodinating activity of neutrophils, which in the case of 1-hp was due to stimulation of the release of myeloperoxidase by activated neutrophils. 1-hp also increased the release of lysozyme by activated neutrophils. Pyocyanin caused only slight enhancement of the release of myeloperoxidase and lysozyme by stimulated neutrophils but was more potent with respect to the release of the specific granule marker, vitamin B12-binding protein. These data indicate the existence of diverse, proinflammatory interactions of pyocyanin and 1-hp with human phagocytes, which may intensify neutrophil-mediated tissue damage during P. aeruginosa infections.