Viability of freeze-dried Serratia marcescens 8UK was studied at 24.5 °C as a function of oxygen concentration and time. Results depended upon the source of the inoculum. Oxygen proved to be toxic and the kinetics (with respect to oxygen concentration) were first order at low oxygen concentrations and zero order at high oxygen concentrations. This indicates that the site for the action of oxygen can become saturated with oxygen. No simple relationship between viability and time was observed. Free radical studies were also made and showed that the detected free radicals were not involved in causing oxygen-induced loss of viability.The survival of Escherichia coli B frozen quickly and stored at −80 °C was found to be little, if at all, influenced by oxygen. Similar results were obtained with Serratia marcescens 8UK. Free radical studies were performed on E. coli B at −80 °C, but no free radicals were detected even after storing E. coli B for 30 days at −80 °C, −20 °C, and −10 °C. Under the last two conditions appreciable loss of viability occurred when E. coli B was frozen slowly.The results indicate that oxygen-induced loss of viability in freeze-dried and frozen and thawed Serratia marcescens 8UK and Escherichia coli B does not involve the formation of free radicals as has been suggested by other workers.