We have investigated the use of a novel technique, in situ end-labelling, as a means of the specific identification of apoptotic cells in formalin-fixed, paraffin-processed tissue sections. The technique relies on the presence of DNA strand breaks in apoptotic cells, caused by activation of endogenous nuclease activity during the process of cell death. These strands are labelled with a non-isotopic reporter molecule in the presence of a DNA polymerase, and labelled DNA is identified immunohistochemically. We show that in situ end-labelling stains cells with the morphological characteristics of apoptosis, and greatly simplifies their identification. Furthermore, in two model systems, the number of labelled cells parallels the number of cells undergoing apoptosis as measured by alternative techniques. The ability of the Klenow fragment of DNA polymerase to label apoptotic nuclei suggests that the characteristic DNA fragmentation seen during this process involves the formation of DNA breaks with a 5' overhang. In situ end-labelling will be valuable for the identification and quantitation of apoptosis in a range of normal tissues and in a variety of pathological states. However, the technique is not specific for programmed cell death, and results must be interpreted with caution and correlated with morphological criteria of apoptosis.