The DNA damaging effect of chlorobenzene was investigated in peripheral lymphocytes and bone marrow cells from C57BL/6 female mice using a gel electrophoresis assay for DNA from single cells ('the single cell gel electrophoresis assay') under alkaline conditions. The effect of chlorobenzene was studied both after single and repeated intraperitoneal injections of 750 mg/kg body weight. The cytostatic agent cyclophosphamide (150 mg/kg, i.p.) was used as a reference substance, and vehicle-treated mice as controls. DNA damage was recorded 16 h after the (last) injection, using an automated computerized image analysis system specifically designed for the single cell gel electrophoresis assay. There was evidence of chlorobenzene-induced DNA damage after 3 days of repeated exposure in peripheral lymphocytes, but no indications of such an effect in bone marrow cells. Cyclophosphamide induced significant damage to DNA both in bone marrow cells and lymphocytes, the effect being most pronounced in the latter cells. It is concluded that high-dose exposure to chlorobenzene is associated with genotoxicity to peripheral lymphocytes. However, this solvent is apparently not a major hazard to bone marrow cells, even after repeated high-dose exposure.