DNA single strand breaks induced by 4NQO or UC-mimetic agents were efficiently accumulated by post-incubation with hydroxy-urea and arabinofuranosyl cytosine (ara C) as compared with those induced by MMS. During the accumulation of 4NQO-strand breaks, various doses of MMS were added and the amount of DNA single strand break was determined by alkaline sucrose gradient sedimentation. The results indicated that the obtained strand break frequencies gradually deviates from the expected additivity of each DNA damage. This was not observed in XP 20S, an incision-defective human cells. All these results suggest that the action of 4NQO damage-specific endonuclease is inactivated by alkylation with high doses of MMS.