Eight monthly semen samples from 45 men not known to be exposed to industrial toxicants were measured by the flow cytometric sperm chromatin structure assay (SCSA). This assay determines susceptibility of sperm DNA to in situ, acid-induced denaturation and is quantitated by the metachromatic shift of acridine orange fluorescence from green (native DNA) to red (denatured DNA). The observed green versus red fluorescence scattergram (cytogram) patterns were generally unique between donors and homogeneous within a donor over time. Within a donor, the cytogram patterns were the same whether intact sperm cells or detached nuclei were measured. For some individuals the cytogram patterns differed for some months and then returned to the original pattern. Intraclass correlations for mean and standard deviation of alpha t [alpha t = red/(red + green) fluorescence] were higher (.67 to .90) than any classically measured semen variables, suggesting that SCSA results within an individual were more consistent than other measures. Furthermore, average within-donor CV of alpha t parameters expressed as a percent of any given individual's means was around 10%, which is significantly lower than those derived from common semen measures. The SCSA is an objective, technically sound, biologically stable, sensitive, and feasible measure of semen quality.