Four photosensitizers with specific targets (mitochondria, lysosomes and plasma membrane) were used to delineate the mechanism of PDT-induced apoptosis in murine leukemia cells. Additional studies were carried out with two sensitizers which caused photodamage to both mitochondria and lysosomes, but varied with regard to membrane photodamage. PDT induced an apoptotic response after mitochondrial photodamage, but not after selective damage to lysosomes or to the cell membrane. Moreover, the latter could delay or inhibit the appearance of apoptosis after mitochondrial photodamage. We had previously reported that exposure of cells to high porphycene concentrations caused an apoptotic response in the dark; this was also associated with mitochondrial damage. These results are consistent with recent proposals that release of mitochondrial components can trigger an apoptotic response. ATP depletion after mitochondrial photodamage does not appear to play a role in initiation of the apoptotic program.