1. The minimum lethal dose of x-rays for Euplotes taylori was determined. Under the conditions of this investigation a 220 second exposure (2110 Roentgen units per second) was required to kill the protozoon. Much less exposure was sufficient to kill the associated bacteria. This difference in resistance permits the sterilization of protozoa with comparative ease.

2. Irradiation of Euplotes for 100 to 220 seconds caused a complete but temporary cessation of ciliary activity in many of the organisms, the percentage so affected increasing with the length of irradiation.

3. Pure cultures of Pseudomonas fluorescens and Bacillus coli, K13, separately irradiated, were found to be killed much more readily than protozoa,—the former in 15 seconds exposure (2530 Roentgen units per second) and the latter in 45 seconds.

4. The death of these organisms by irradiation was not due to the action of toxic products in the medium since separately irradiated media were not found to be toxic.

5. Irradiated bacteria were found unsatisfactory for the nutrition of Euplotes, previously sterilized either by irradiation or washing.

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