Photosensitization using the tumor-localizing porphyrin Photofrin induces cell death both in vitro and in vivo, but the mechanism of cell death is not well understood. Cell lysis (necrosis) and apoptosis have both been observed. The latter seems restricted mainly to lymphoma and epithelial cell lines. To check the influence of the incubation protocol on the cell death mechanism, CV-1 cells were loaded with Photofrin using two different protocols. In both protocols, photosensitized CV-1 cells underwent severe morphological changes before cell death. Many cells treated with protocol 1 (24 h with 1 microgram/mL of Photofrin in culture medium) underwent apoptosis, as demonstrated by plasma membrane blebbing and fragmentation into vesicles, condensation of the chromatin and fragmentation of the nucleus with oligonucleosomic degradation of the DNA. In contrast, cells treated with protocol 2 (1 h with 10 micrograms/mL of Photofrin in phosphate-buffered saline) lysed instead of fragmented, without oligonucleosomic degradation of the DNA. This type of cell death looks much like necrosis. However, early morphological changes suggest that it is, in fact, apoptosis stopped by plasma membrane leakage. It is concluded that apoptosis is primarily induced in CV-1 cells but may be arrested by membrane lysis, depending on the incubation protocol.