CHO cells were pulse-treated for 20 min with increasing doses of streptonigrin (SN). First division cells analyzed 18 h after the end of treatment showed a combination of chromosome and chromatid aberrations with a typical dose-response curve. The frequency of SCEs in second division cells (24 h harvesting time) also exhibited a positive correlation with the SN dose. A high incidence of chromatid aberrations was detected in second and third division metaphases. This indicates that SN has a persistent clastogenic action that lasts for at least three cell cycles after the end of treatment. The kinetics of DNA damage and repair was studied by the alkaline unwinding and single cell gel methods. It was found that a pulse treatment with SN elicited a triphasic response characterized by repair-damage-repair. It is proposed that SN forms stable complexes with the DNA. These complexes by a continuous cycling redox process would produce active oxygen species inducing direct damage to DNA.