DNA damage in single peripheral blood (pb) and bone marrow (bm) cells was studied in dogs which were exposed to total body X-ray irradiation (TBI) with a lethal dose of 3.9 Gy. The changes in pb and bm cell numbers were measured within 9 days after TBI. Using the alkaline single-cell gel electrophoresis technique ('comet' assay), DNA strand breaks and alkali labile sites were assessed in single cells derived from the blood before TBI, 1 h and 4 h after TBI and on days 1, 3 and 9 after TBI. Bone marrow cells subjected to the assay were collected before and on days 1 and 9 after TBI. Cells expressing the strongest DNA damage were most frequent in the blood 1 h after TBI and in the bone marrow 1 day after exposure. Thereafter, a continuous reduction of DNA damage in individual cells was observed in the course of progressive leukopenia and granulocytopenia.