In approximately 5% of cancer patients undergoing radiotherapy, this treatment has to be interrupted because of an acute reaction of normal tissues. To test the possibility of predicting this type of reaction, the micronucleus assay was used to determine radiosensitivities of peripheral blood lymphocytes of 15 patients with severe acute reaction of normal tissue, 15 patients without this reaction and 15 healthy donors. Whole-blood cultures were irradiated with X-rays (4 Gy, 1.08 Gy/min) and treated with cytochalasin B. The micronuclei scores observed in irradiated cells were corrected for the scores in unirradiated cells. Intra-individual and interindividual variations in micronuclei scores were analysed in samples from healthy donors, and highly significant interindividual differences were found (P < 0.001). Scores of cells not irradiated in vitro were higher for cancer patients before radiotherapy than for healthy donors (P < 0.001), and those for cancer patients after radiotherapy were higher than for patients before radiotherapy (P < 0.001). Average micronuclei scores induced by in vitro irradiation were significantly higher in samples from cancer patients compared with those from healthy donors (P < 0.01). Moreover, all subgroups of cancer patients included individuals with very high levels of micronuclei after in vitro irradiation. There was, however, no relationship between the micronuclei scores and the occurrence of severe acute reactions in normal tissues.