BACKGROUND: Evaluation of radiation-induced apoptosis in T-lymphocytes was developed for human medicine in order to predict the sensitivity of individual patients to radiation therapy and has regular use in cases of suspected hypersensitivity. A major goal of the present study was to evaluate the usefulness of the apoptosis assay in veterinary medicine for application in radiation sensitivity testing. The main goal was to examine potential changes in sensitivity of T-lymphocytes to radiation-induced apoptosis during the course of radiation treatment. This is a clear example of the advantageous use of spontaneous canine tumors to augment human cancer research. MATERIAL AND METHODS: Blood was collected in heparin tubes, diluted 1 : 10 in RPMI medium, irradiated with X-rays and incubated for 48 h. T-lymphocytes were labeled using FITC-conjugated antibodies, erythrocytes were lysed, and DNA stained with propidium iodide. For cell analysis, a Becton Dickinson FACScan flow cytometer was used. Radiation-induced apoptosis in T-lymphocytes was quantified. Blood samples from tumor-bearing dogs were taken before the first fraction and at the end of radiation therapy. RESULTS: Apoptosis in lymphocytes is dependent on donor age and donor weight. Tumor-bearing dogs when compared with healthy dogs showed no significant differences in levels of induced apoptosis. No significant changes were seen in the levels of radiation-induced apoptosis in blood taken before, during, or after radiation therapy. CONCLUSION: The leukocyte apoptosis assay can be successfully applied to canine patients, and a wide spectrum of sensitivities to radiation-induced apoptosis is observed. The sensitivity of a patient's peripheral blood T-lymphocytes to radiation-induced apoptosis does not change as a result of the trauma of radiotherapy during the course of tumor treatment.