This paper reports that the Lw value (kb) of HpaⅡ digests of cellular DNA labelled with ³H-TdR before harvest was not changed in the surviving FL cells after MNNG treatment. While in FL cells which had been prelabelled with ³H-TdR and then treated with MNNG or Hanks so-lution, the Lws of DNA from detached cells were significantly lower than that of the control. After HpaⅡ digestion, the Lws of restriction fragments were remarkably lower than that of the controls. The Lws of HpaⅡ digests of the mixed DNAs of the detached cells were lower than that of the control, after Hanks solution treatment with the control cells and those of the mixed DNAs of surviving cells with the detached cells after MNNG exposure were also lower than that of the controls. Thus, it has been clearly demonstrated that the presence of degenerating cells whose DNA has been broken by autolysis and/or chemical mutagen exposure could affect the result of DNA methylation when specific endonuclease cleavage site analysis is adopted. So the conclusion about cellular DNA hypomethylation could be reached only through experimental design that could restrict the object to be analyzed only in newly-replicated cellular DNA and rule out the unstable and unheritable DNA hypomethylation.