The effects of quorum sensing signal molecules in Pseudomonas aeruginosa, N-butanoyl-L-homoserinelactone (C4-HSL) and N-(3-oxododecanoyl)-L-homoserinelactone (3-oxo-C12-HSL) on planktonic cell resistance against hydrogen peroxide were studied. In P. aeruginosa JP2 cells with the deletion of lasI and rhlI, the viable cell concentration decreased with time and was reduced by about 4 log after 2 h of 7.5 mM H2O2 treatment, while only a 2-log reduction was found for the wild type P. aeruginosa PAO1 cells. When cultured with 20% PAO1 spent medium, P. aeruginosa JP2 showed similar hydrogen peroxide resistance to that seen in P aeruginosa PAO1. Culturing with 20% JP2 spent medium or with 10 microM C4-HSL and 20 microM 3-oxo-C12-HSL did not affect P aeruginosa JP2 cell susceptibility to hydrogen peroxide. Although both 20% PAO1 and JP2 spent media reacted with H2O2 and reduced H2O2 to 50% of the strength of the original concentration, the remaining H2O2 was still sufficient to kill P. aeruginosa JP2. These results indicate that the difference in cell resistance against H2O2 between P. aeruginosa PAO1 and JP2 was related to the existence of gene products of the lasI and rhlI systems. However, adding synthetic homoserine lactones alone did not increase P. aeruginosa JP2 cell resistance to H2O2 as seen in the experiments adding PAO1 spent medium. Determination of the detailed relation between cascade regulation in P. aeruginosa and its cell resistance to H2O2 will require further investigation.