The mitochondrial localization of p53 is an important event in p53-dependent apoptosis. Some p53 mutants defective for transcription also facilitate apoptosis through changes of the mitochondria. Here, apoptosis of HeLa and CaSki cells (p53(wt)), C33A and HaCat cells (p53(mt)) and SaOs-2 cells (p53 deficient) was induced by 300 nM staurosporine. We showed that wild-type p53, as well as p53 mutants, were transiently located to the mitochondria with changes in the mitochondrial membrane potential (Delta Psi m). However, in C33A cells harboring a p53 mutated on its DNA binding domain, Delta Psi m collapse and Sub-G1 DNA content were reduced compared to p53(wt) cells, whereas no significant difference was observed in HaCat cells with a p53 mutated on UV hot spots. In addition, inhibition of the mitochondrial permeability transition pores by cyclosporine A significantly reduced the Delta Psi m loss and the sub-G1 DNA content in p53 positive cells. These results indicate that Delta Psi m collapse is an early and necessary event, which plays an important role in apoptosis of immortal mammalian cells.