The APO-DIRECT™ Kit is a 2-color staining method for labeling DNA breaks and total cellular DNA to detect apoptotic cells by flow cytometry. The kit contains the instructions and reagents required for measuring apoptosis in cells including positive and negative control cells for assessing reagent performance washing, reaction and rinsing buffers for processing individual steps in the assay terminal deoxynucleotidyl transferase enzyme (TdT), Fluorescein-deoxyuridine triphosphate and propidium iodide/RNase A solution for counterstaining the total DNA. One of the most easily measured features of apoptotic cells is the break-up of the genomic DNA by cellular nucleases. These DNA fragments can be extracted from apoptotic cells and result in the appearance of DNA laddering when the DNA is analyzed by agarose gel electrophoresis. The DNA of non-apoptotic cells that remains largely intact does not display this laddering on agarose gels during electrophoresis. The large number of DNA fragments appearing in apoptotic cells results in a multitude of 3'-hydroxyl termini in the DNA. This property can be used to identify apoptotic cells by labeling the 3'-hydroxyl ends with directly conjugated fluorescein- deoxyuridine triphosphate nucleotides (FITC-dUTP). The enzyme terminal deoxynucleotidyl transferase (TdT) catalyzes a template-independent addition of deoxyribonucleoside triphosphates to the 3'-hydroxyl ends of double- or single-stranded DNA with either blunt, recessed or overhanging ends. A substantial number of these sites are available in apoptotic cells providing the basis for the method utilized in the APO-DIRECT™ Kit. Non-apoptotic cells do not incorporate significant amounts of the FITC-dUTP due to the lack of exposed 3'-hydroxyl DNA ends. Sufficientreagents are provided to process 50 cell suspensions including 5 ml positive and 5 ml negative control cell suspensions of approximately 1x106 cells per ml in 70%(v/v) ethanol.