To investigate the relationship between differentiation and the rejoining ability of DNA strand breaks, DNA strand breaks induced by gamma-rays were analyzed in barley roots using the alkaline unwinding assay. The extent of unwinding in an alkaline solution containing 0.5 M NaCl was found to be significantly inhibited at the root tip consisting of meristematic cells but not in the remainder of the root consisting of differentiated cells immediately after 100 Gy of gamma-irradiation. The difference in the extent of unwinding was diminished when the alkaline solution contained 2 M NaCl, suggesting a difference of chromosome structure and/or cell skeleton between the two regions. The rejoining kinetics of DNA strand breaks consisted of a fast and a slow component in both regions. DNA strand breaks were rejoined to the level of unirradiated control at the root tip but remained partly unrejoined in the remainder of the root during 6 h post-irradiation incubation. The difference of rejoining ability observed between the two regions originated from the different efficiency of rejoining at the slow component. The rejoining at the slow component in the root tip was found to be inhibited in the presence of the protein synthesis inhibitor cycloheximide, suggesting that de novo synthesized proteins are involved in the rejoining of DNA strand breaks. In contrast, the slow component in the remainder of the root was not inhibited by cycloheximide. These results suggest that the reduced rejoining ability of DNA strand breaks of differentiated cells may result from a deficiency of rejoining DNA strand breaks by inducible repair at the slow component. In addition, the lack of an apparent correlation between rejoining ability and growth inhibition of the root is discussed.