An apparatus combining continuous fast-flow mixing with rapid irradiation has been used to study the time scale of the oxygen effect in irradiated bacteria and mammalian cells. The range of times used was from 2 to 40 msec between adding O2 and irradiating, either before or after mixing. The effect of adding O2 to bacteria either before or after irradiating is dependent on whether the bacteria are suspended in broth or in buffered solution. When O2 is added before irradiation, larger enhancement ratios are obtained with the bacteria suspended in broth rather than in buffer. When the O2 is added 5 msec after irradiation, there is a small degree of sensitization in buffer but none in broth. In Chinese hamster cells, the oxygen effect is resolved into two time-dependent components. For O2 added to these cells 3-5 msec before irradiation, the oxygen enhancement ratio (OER) is 1.7 and is independent of the O2 concentration between 1 and 50%. As the time interval from addition of O2 to irradiation is increased from 5 to 40 msec, the OER increases in a manner dependent upon O2 concentration. On addition of 50% O2 5 msec after irradiation, only a very small increase in sensitivity is observed. The time-resolved oxygen effects are discussed in terms of possible mechanisms.

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