Background: Cytochrome c release from mitochondria to cytosol is a hallmark of apoptosis and is used to characterize the mutochondria-dependent pathway of this type of cell death. Techniques currently used to measure cytochrome c release, Western blot and fluorescence microscopy of imnumolabeled cells, are time-consuming and inaccurate. and the latter is still limited by sample size. Methods: We developed a rapid and reliable technique to detect cytochrome c release during drug-induced apoptosis, using flow cytometry. Plasma membrane of apoptotic HL-60 cells and thymocytes, treated with staurosporine and dexamethasone. respectively, were selectively permeabilized by digitonin at a low concentration. The released cytochrome c vas quickly washed out from cells and that which remained in the mitochondria was immunolabeled after fixing the cells. Results: The fraction of cells that retained their mitochondrial cytochrome c, or the highly fluorescent cells, gradually decreased so that after 4-8 h of drug treatment almost all the cells lost their cytochrome c and emerged as a population of low fluorescent cells. This was confirmed by parallel fluorescence microscopy of cells immunolabeled for cytochrome c. Conclusions: This technique allows the analysis of cytochrome c release from mitochondria of a large number of apoptotic cells in a short period of time and is proposed as an alternative to the methods Currently used for this same purpose.(c) 2006 International Society for Analytical Cytology.69A6515523