The effects of 5-aza-2'-deoxycytidine on DNA in mouse L1210 leukemia cells were investigated using the alkaline elution technique. By comparing the DNA elution rate at pH 12.1 and 12.6, it was found that the drug produced DNA alkali-labile lesions. Alkali-labile sites were present only in DNA strands that were synthesized in the presence of the drug. They persisted for at least 48 h after drug treatment, and only after 72 h did the number of alkali-labile sites decline, thus suggesting a slow repair process. The production of alkali-labile sites was found to be concentration dependent and observable at concentrations which were effective in inhibiting the clonogenic viability of L1210 cells and which are attainable in vivo. 5-Aza-2'-deoxycytidine did not cause other DNA lesions such as DNA double-strand breaks or DNA-protein cross-links. Two hypotheses were considered to explain the origin of alkali-labile lesions in DNA that has incorporated 5-aza-2'-deoxycytidine: (a) the production of apyrimidinic sites by a glycosylase that recognizes and removes aza-cytosine from DNA and (b) the alkali-catalyzed decomposition of azacytosine residues to ring-opened products which could lead to alkali-induced DNA strand scission through a beta-elimination mechanism. The second hypothesis was considered to be the more probable and suggests that the alkali lability may be a means by which one could determine the extent of substitution and precise location of azacytosine residues or their ring-opened products in DNA.