The presence of sperm cells characterized by increased sensitivity of DNA in situ to denaturation and by abnormal morphology (shape) in semen is associated with low fertility. Such cells often appear in increasing numbers following exposure to toxic or mutagenic agents. In the present study, the sensitivity of DNA in situ to acid-induced denaturation was assayed in sperm cells of 25 patients attending a human fertility center. The same samples were also subjected to analysis of DNA strand breaks. A new method for detecting DNA strand breaks in situ by labeling 3'-OH termini with biotinylated dUTP in a reaction employing exogenous terminal deoxynucleotidyl transferase has been applied. Both methods utilized flow cytometry. A good correlation was observed in all samples between the percentage of sperm cells with DNA strand breaks and the percentage of cells characterized by an increased sensitivity of DNA to denaturation (r = 0.87; P < = 0.05). It is likely, thus, that the sperm cells characterized by increased DNA sensitivity to denaturation have extensive DNA strand breakage. The above properties of abnormal sperm cells, which in all probability are dead in terms of their reproductive capacity, resemble properties of apoptotic somatic cells. Activation of the endogenous endonuclease which causes extensive DNA breakage, thus, is characteristic to both the programmed death of somatic cells (apoptosis) and functional elimination (of possibly defective) germ cells from the reproductive pool.