The frequency of micronuclei resulting from chromosome breaks, and that of micronuclei deriving from spindle disturbances was determined in exfoliated cells of the human buccal mucosa in 50 normal individuals. Several confounding factors, such as age, smoking habits, alcohol consumption, etc., were taken into account. While the frequencies of micronuclei resulting from chromosome breaks and of cells with this kind of micronuclei were about double in smokers as compared with non-smokers, the difference being highly statistically significant, the frequencies of cells with spindle disturbances were almost the same in the two groups. No statistically significant correlation was found for any of the other variables examined. In two patients suffering from cancer of the oral cavity the variation of the frequency of the micronuclei during the progress of radiotherapy and following its interruption was determined. It was found that gamma rays induced only micronuclei resulting from chromosome breaks, whose frequency increased linearly with the applied dose, and was lowered to the initial background level 7-12 days after the end of radiotherapy.

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