The frequency of single-strand breaks in parental DNA and gaps in nascent DNA in various cells exposed to methyl methanesulfonate (MMS) or methylnitrosourea (MNU) was investigated by alkaline unwinding assay using two types of alkaline lysis conditions, 22 degrees C lysis versus 0 degrees C lysis. The DNA damage induced by MMS and MNU is considered to be characteristic of lesions produced in DNA by alkylating agents. The aim of our research project was to adjust this method to be able to detect the greatest number of DNA lesions induced by alkylating agents in parental DNA of different mammalian cells. In our experiments we used human cell lines EUE, GM637 and XP12, Chinese hamster V79 cells, and Syrian hamster embryo cells. The higher level of strand interruptions was detected under conditions of lysis of cells at 22 degrees C. Probably the level of strand interruptions found after the lysis of cells at 22 degrees C correlates with the increased number of disrupted alkali-labile sites of DNA. It is remarkable that the different lysis conditions did not influence the number of gaps detected in nascent DNA of alkylated cells. Comparing induction of breaks and gaps in radiolabelled strands of parental and daughter DNA under different lysis conditions, we succeeded in defining the optimum conditions for detection of alkali-labile sites of parental DNA.