Apoptosis or programmed cell death plays a pivotal role in embryonic development and maintenance of homeostasis. It is also involved in the etiology of pathophysiological conditions such as cancer, neurodegenerative, autoimmune, infectious and heart diseases. Consequently, the study of apoptosis is now at center of both basic and clinical research applications. Therefore sensitive and simple apoptosis detection techniques are required. This study involves identification and characterization of a monoclonal antibody-defined novel antigen, namely NAPO (negative in apoptosis), which is specifically lost during apoptosis. The anti-NAPO antibody recognizes two nuclear polypeptides of 60 kD and 70 kD. The antigen is maintained in quiescent and senescent cells, as well as in different phases of the cell cycle including mitosis. Thus, immunodetection of NAPO antigen provides a specific, sensitive and easy method for differential identification of apoptotic and non-apoptotic cells.