SUMMARY:Resistance values were determined for spores ofBacillus coagulans(var. thermoacidurans), conidiospores ofAspergillus niger, vegetative cells ofLactobacillus brevis, Leuconostoc mesenteroides, Hansenula anomala and Saccharomyces cerevisiae(var. tetrasporus) exposed to a mixture of ethylene oxide (12%)and dichlorodifluoromethane (88%). Exposures were made of organisms deposited on glass and paper discs at 33% relative humidity, 30°C and a pressure to give 700 mg per liter of ethylene oxide. Spores ofB. coagulanswere also exposed at 40°, 50° and 60°C. The resistance parameter D was determined to characterize the resistance of each organism displaying logarithmic death. Values of z were determined for spores ofB. coagulans.The most resistant organism wasB. coagulans.The spores of this organism were approximately 1.8 to 8 times more resistant than the vegetative cells ofL. mesenteroidesorL. brevis.Vegetative cells ofH. anomala, S. cerevisiaeand conidiospores ofA. nigerdisplayed nonlogarithmic death. A. niger was the most resistant of the th