Polysome content and rate of amino-acid incorporation have been measured in exponentially growing cultures of the ciliate protozoan Tetrahymena pyriformis after doses of 0-120 krad of60 Co gamma-radiation delivered at three different dose rates. The polysome content decreases exponentially with increasing dose at all three dose rates. Curiously enough, the sensitivity to dissociation was lower at the intermediate dose rate than at either the high or low one. Simple target theory analysis of the data indicates a target size for polysome dissociation in the range between a single ribosome and a disome. Amino-acid incorporation is more sensitive to radiation than is polysome content. Data obtained at the highest dose rate suggest that there is a second sensitive target which is considerably smaller than the more sensitive one.

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