The effect of N2O on eight strains of bacteria was measured in dilute suspensions. The dose-modifying factors (DMF) of N2O on M. radiodurans R1, P. radiora 0-1, M. lysodeikticus and B. pumilus E601 (vegetative cells) were 3 . 4, 2 . 9, 2 . 4 and 1 . 7, respectively. But P. radiora RP-C, P. fluorescens B3-1, E. coli B/r and E. coli K-12 were hardly sensitized by N2O. From measurements of catalase activity of each bacterium, it was found that the DMF increases with increased catalase activity, suggesting that cellular catalase promotes the sensitizing action of N2O.