The most widely used histochemical marker of apoptosis (in situend labeling, TUNEL) detects both apoptotic and necrotic cells and evaluates only late stages of apoptosis. Hence, a specific and sensitive cellular marker of apoptosis is needed to determine the role of apoptotic death in biology and pathology. The present study describes a novel immunohistochemical procedure for the staining of apoptotic cells using a monoclonal antibody (MAb) to single-stranded DNA. This MAb stained all cells with the morphology typical of apoptosis in etoposide-treated HL-60, MOLT-4, and R9 cell cultures, in which apoptosis was accompanied by high, moderate, and low levels of internucleosomal DNA fragmentation, respectively. TUNEL stained all apoptotic cells in HL-60 cultures, nearly 60% of apoptotic cells in MOLT-4 cultures, and only 14% of apoptotic cells in R9 cultures. Apoptotic R9 cells, which progressed into secondary necrosis, retained MAb staining and became TUNEL-positive. Necrotic cells in MOLT-4 cultures treated with sodium azide were stained by TUNEL, but were negative for MAb staining. All floating cells at a late stage of apoptosis in MDA-MB-468 cultures treated with cisplatin were stained by both MAb and TUNEL. However, among adherent cells in the early stages of apoptosis, MAb stained nearly 20 times more cells than TUNEL. In histological sections of human tumor xenografts, MAb detected clusters of apoptotic cells in viable tumor tissue, but did not stain cells in areas of central ischemic necrosis. In contrast, TUNEL stained nuclei in necrotic areas. Thus, MAb to single-stranded DNA is a specific and sensitive cellular marker of apoptosis, which differentiates between apoptosis and necrosis and detects cells in the early stages of apoptosis.