Hydrogen peroxide is used in various applications to prevent, control or decrease bacterial activity in e.g. cooling water, hospitals, recreational waters and the food industry. The aim of the work reported here was to investigate the response of P. aeruginosa following exposure to hydrogen peroxide during both the logarithmic and stationary phases of growth. The catalase levels were determined following exposure to hydrogen peroxide and the general cellular response was investigated by pulse-labelling. Stationary phase cells did not demonstrate a stress response to hydrogen peroxide. Where de novo protein synthesis was inhibited, cells were less susceptible to growth inhibition, indicating an adverse stress response to hydrogen peroxide in P. aeruginosa. The addition of hydrogen peroxide to cultures in logarithmic growth phase resulted in the induction of a short lag phase. The growth rate following a return to logarithmic growth phase was lower than before addition of hydrogen peroxide, and was inversely related to the concentration of hydrogen peroxide added. Oxidising stress elicited de novo synthesis of four proteins within 5 min following exposure to stress. Cellular catalase levels doubled from 16 U/mg protein to over 30 U/mg within 10 min following exposure to oxidising stress but no new catalase isozymes were induced. Hydrogen peroxide was demonstrated to interrupt cell division as well as to decrease the ensuing rate of division in P. aeruginosa, and the culture did not exhibit an effective stress response to hydrogen peroxide.