To investigate the dose-dependent effects of neutral oxygen radicals on the proliferation as well as the inactivation of microorganisms, we treated suspensions of budding yeast cells with oxygen radicals using an atmospheric-pressure oxygen radical source, varying the fluxes of O(3Pj) from 1.3 × 1016 to 2.3 × 1017 cm−2 s−1. Proliferation was promoted at doses of O(3Pj) ranging from 6 × 1016 to 2 × 1017 cm−3, and suppressed at doses ranging from 3 × 1017 to 1 × 1018 cm−3; cells were inactivated by O(3Pj) doses exceeding 1 × 1018 cm−3, even when the flux was varied over the above flux range. These results showed that the growth of cells was regulated primarily in response to the total dose of O(3Pj).