We developed an improved method for the detection of double-strand DNA breaks in apoptotic cells at both the light (LM) and electron microscopic (EM) levels using a modification of the TdT (terminal deoxynucleotidyl transferase)-mediated dUTP nick end-labeling (TUNEL) technique. Cultured rat cerebellar granule cells were exposed to low potassium conditions to induce apoptosis. Twenty-four hr after treatment, one group of cells was fixed in situ with 4% paraformaldehyde and labeled for DNA fragmentation characteristic of apoptosis. Apoptotic cells were visualized with diaminobenzidine (DAB) and viewed by LM. The second group of cells was detached from the culture dish, pelleted, fixed with a 4% paraformaldehyde and 0. 2% glutaraldehyde mixture, and embedded in LR White. For LM, the modified TUNEL technique was performed on 1.5-microm LR White sections and apoptotic cells were visualized using an enzymatic reaction to generate a blue precipitate. For EM, thin sections (94 nm) were processed and DNA fragmentation was identified using modified TUNEL with streptavidin-conjugated gold in conjunction with in-depth ultrastructural detail. Alternate sections of cells embedded in LR White can therefore be used for LM and EM TUNEL-based detection of apoptosis. The present findings suggest that the modified TUNEL technique on LR White semithin and consecutive thin sections has useful application for studying the fundamental mechanism of cell death. (J Histochem Cytochem 47:561-568, 1999)