A group of albino rats, Sprague-Dawley strain, were exposed to x-rays at levels of 1, 3, 10, 30, and 100 R in single or weekly doses to determine the sensitivity of the response of blood lymphocytes to ionizing radiation in terms of their size distribution. The cells were categorized as large (>12 microns) or small (<12 microns) on the finding that the median diameter of the lymphocyte in the normal animal was 12 microns (range 8.5 to 17 microns) as measured with a calibrated ocular micrometer at 1600× on Wright-Giemsa-stained blood smears. The ratio of the sum of large lymphocytes and monocytes to small lymphocytes sized in random fields was used as an index of response. The lymphocyte ratios of the animals exposed only once to 30 R or less were not significantly different by Student's t-test from control values. The dose-ratio relationship was exponential for repetitive exposures, while that for single irradiations was not at all clear. The possible use of this indicator of response as a biological dosimeter and thus for determination of the biological equivalence (relative biological effect) of other types of ionizing radiation appears feasible under the experimental conditions used in this study.

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