The Mitochondria/Cytosol Fractionation Kit provides unique formulations of reagents for effective isolation of a highly enriched mitochondrial fraction from cytosolic fraction of mammalian cells including both apoptotic and nonapoptotic cells. The enriched mitochondrial and cytosolic fractions can be used for studying apoptotic and signal transduction pathways to detect translocation of factors interested between the two fractions by Western blotting, ELISA, or other assays. Procedures are simple and easy to perform, no ultracentrifugations and toxic chemicals are involved.