A double-labeling assay for detecting apoptotic cells, using the terminal deoxynucleotidyl transferase (TdT)-mediated dUTP nick end-labeling (TUNEL) assay, and antigens of interest, using immunofluorescence, is described. The assay has been used successfully on fixed, cultured cells and formalin-fixed paraffin-embedded tissue sections. This in situ detection system can be used to correlate apoptosis with specific antigen expression to identify factors involved in the induction and execution of the apoptotic pathway of cell death in a wide variety of experimental systems.