The induction of sister-chromatid exchanges (SCEs) by urethane, 150 and 300 mg/kg administered i.p., was examined in bone-marrow cells of AKR, BALB/c, C3Hf, C57BL/6J and DBA/2 male mice. In all strains, the base-line level of SCE/cell was similar, ranging from 4.3 to 8.7, and the response increased with the dose of urethane. DBA/2 mice were the most susceptible to urethane at both dose levels, with 30.6 SCE/cell after treatment with 300 mg/kg, whereas the response of the other strains was from 17.4 to 21.5 SCE/cell at the same urethane dose. Pre-treatment of C57BL/6J and DBA/2 mice with phenobarbital decreased the SCE frequencies induced by urethane, 300 mg/kg, to 70%, whereas a prior administration of β-naphthoflavone reduced SCE levels in C57BL/6J but not in DBA/2 mice.
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