The alkaline unwinding assay was applied to measure strand breaks in DNA of intact barley roots after gamma-ray irradiation without extracting DNA from the tissue. The most suitable condition for unwinding was determined to be 0.03 M NaOH containing 0.5 M NaCl for alkaline solution composition, and 15 min at 20 degrees C for unwinding time. Under this condition, DNA strand breaks were shown to be linearly increased with increasing dose, and further be rejoined during the post-irradiation incubation for 6 hr.