The ability of methyl methanesulfonate (MMS) to induce DNA breakage in spermiogenic stages of the mouse was studied using an alkaline elution technique. At daily intervals over a 3-week period following i.p. injection of 50 mg MMS/kg, mature spermatozoa were recovered from treated (3H-labeled) and control (14C-labeled) animals, lysed together on polycarbonate filters, and eluted with a high pH (12.2) buffer. Elution of germ-cell DNA from MMS-treated animals was found to increase in stages in which genetic damage from MMS is greatest. In general, the pattern of DNA elution from treated, spermiogenic stages paralleled the pattern of sensitivity to dominant lethals, specific-locus mutations and heritable translocations found by other investigators. It also paralleled the pattern of sperm-head methylation and protamine methylation measured in an earlier study (Sega and Owens, 1983). At 9 days post treatment (sperm sampled were in mid-to late-spermatid stages at the time of MMS exposure) the elution of sperm DNA did not change significantly over a pH range of 11.6-12.8, suggesting that, at the time of assay, DNA breaks were already present in the sperm. Because of the parallelism found between increased sperm DNA elution and increased genetic damage after mutagen treatment, alkaline elution may prove useful in monitoring potential genetic damage in human sperm.