Alkaline single-cell gel electrophoresis (the 'comet assay') was used to evaluate DNA damage in lymphocytes from 17 breast cancer patients before and 1-21 h after chemotherapy including cyclophosphamide (600-1800 mg/m2). In order to control for the experimental variability over time, freshly isolated lymphocytes from female mice given physiological saline or cyclophosphamide (150 mg/kg b.wt.) were included as 'internal standards' in each individual electrophoresis run. There was an upward tendency of DNA damage in the mouse lymphocytes over the study period, but cyclophosphamide was constantly found to induce significant damage at all time points investigated (1-48 h). Although patients given up to 11 prior cycles of chemotherapy showed the same basal level of DNA damage as the patients coming to the clinic for their first treatment, the chemotherapy given at the time of the present blood sampling was associated with significant DNA damage in most samples. Considerable interindividual variations were observed both before and after the treatment. DNA single-strand breaks and alkali-labile sites in peripheral lymphocytes as evaluated by the comet assay seem to be useful molecular biomarkers for exposure to DNA damaging agents when monitoring ongoing exposures, but less impressive when monitoring accumulated exposures, at least in patients given high doses of cyclophosphamide and other antineoplastic agents.