Cell lines resistant to IUdR and 6-thioguanine were isolated from a radiosensitive mouse lymphoma line LS and its radioresistant derivative AII respectively. Their biological and biochemical characteristics, as measured by plating in HAT medium IUdR and TG and by (14C) hypoxanthine and (3H) thymidine uptake were measured and found to be consistent with those expected for drug resistant cell lines. Two hybrid cell lines were isolated from crosses between the LS IUdRr line and AII TGr line and their radiosensitivity measured relative to that of the parental lines. Radioresistance was found to be dominant in both the hybrid lines as was resistance to EMS and U.V. to which the parental lines also show differential sensitivity. The ability to recover from sublethal damage after X-irradiation was found to be temperature sensitive in radiosensitive cells, an inverse effect being seen when LS cells were irradiated at 20 degrees C and held at 20 degrees C between dose fractions. Hybrid cells showed a normal amount of recovery from sub-lethal damage when compared with that in AII cells but this occurred at t slightly slower rate at 20 degrees "c than at 37 degrees C. Recovery from sub-lethal damage in AII cells was unaffected by lowering the temperature. These findings suggest that the radiosensitive cells may be temperature sensitive, in some step or steps in the ligation process.