A tumour sensitivity assay, the differential staining cytotoxicity (DiSC) assay, which has been shown to have potential in predicting tumour response to cytotoxic drugs, has been used to investigate the radiosensitivity of peripheral blood lymphocytes isolated from patients with chronic lymphocytic leukaemia (CLL). The isolated lymphocytes were irradiated and incubated for 4 days. Radiation-induced cell kill was assessed by differential staining of dead and live cells with subsequent calculation of tumour cell viability. The results of 61 CLL specimens from 40 patients are reported, showing profound inter-patient differences in the sensitivity of cells to radiation. Five patient specimens, which were radioresistant in vitro, were from patients who were also resistant clinically to irradiation. Another patient who responded very well clinically was found to be extremely sensitive in the assay.