['A recent report by Macho et al. (Cytometry 25: 333-340, 1996) described the use of chloromethyl-X-rosamine (CMX-Ros) as a fixable probe for detection of loss of mitochondrial membrane potential (psi(mit)), an early event in many models of apoptosis. However, this previous report lacked a description of any direct comparisons between pre- and post-fixation analyses of normal and apoptotic cells stained with CMX-Ros.', 'Using a variety of cell types, we investigated the effect of paraformaldehyde fixation on cellular retention of CMX-Ros and the implications of this for the subsequent analysis of changes in psi(mit) in cells undergoing apoptosis.', 'We found that following fixation, the resolution between normal cells with polarized mitochondria and apoptotic cells with depolarized mitochondria is reduced to the extent that accurate discrimination between the cell types is no longer possible.'] Is the use of chloromethyl-X-rosamine ( Mitotracker red ) to measure loss of mitochondrial membrane potential in apoptotic cells incompatible with cell fixation?