A methodology for quantifying DNA double-strand breaks in human sperm is described. Sperm from three healthy human donors on three separate days each were irradiated with 12.5, 25, 50 and 100 cGy X-rays. Linear dose-response effects were observed in migrated DNA from sperm nuclei when electrophoresed under neutral conditions. RNase and proteinase K treatments for longer duration were necessary, to decondense the chromatin and presumably to release the broken DNA for migration in the electrophoretic field in a dose-dependent manner. An increase in DNA migration was observed with as low as 12.5 cGy, but damage was observed in all samples at 25 cGy. No evidence of repair of these X-ray-induced DNA double-strand breaks was observed during a 2 h period.