[en] Purpose: To present the effects of different radiotherapeutic treatments on the morphonuclear characteristics and growth of the MXT mouse mammary adenocarcinoma. Methods and Materials: We collected MXT tumor cells by means of fine-needle aspirations during various radiotherapeutic treatments and analyzed the morphological aspects of the cell nuclei by means of the digital cell image analysis of Feulgen-stained nuclei. In addition, we studied the morphonuclear aspects of cells from MXT tumors that had been radioresistant cell enriched. These radioresistant cell-enriched tumors involved MXT tumors that had survived one or two previous radiotherapies. The radiotherapy-induced effects on the morphonuclear characteristics were monitored by means of both monovariate (one-way variance) and multivariate (principal components and step-wise linear discriminant) analyses. Results: The monovariate analyses showed that radiotherapy significantly influenced the values of the parameters relating to nuclear size (nuclear area-NA), the frequency of small dense chromatin clumps (short run length emphasis-SRL) in the nuclei, and the overall chromatin condensation level (local mean-LM). The global effect corresponded to a decrease in the overall chromatin condensation level in the radioresistant cell-enriched MXT tumors. This decrease occurred concomitantly with an increase in the frequency of the small dense chromatin clumps in the nuclei and a decrease in the nuclear area. The multivariate analyses showed that it was possible to quantitate the proportion of 'radiosensitive-like' and 'radioresistant-like' cell nuclei in the various MXT tumor types under study. Conclusions: The development of certain morphonuclear parameters, that is, the NA, the SRL, and the LM, could be proposed to predict the response of human tumors to radiotherapy as, indeed, could the quantitation of the proportion of radioresistant cells