The measurement of time correlated intracellular mitochondrial staining with 3,3'-dipentyloxacarbocyanine [Di-O-C5(3)] appeared of interest to define the optimal staining conditions. Mitochondrial staining of lymphocytes, monocytes and granulocytes results in different fluorescence signals, related to the numbers of mitochondria, that are present in the cells of these various cell types. Alterations of Di-O-C(5)3 staining in a distinct cell type are due to changes in the physiological or functional state of the mitochondria. It appeared that such alterations occur in cells, which are cultured in the presence of cytosine arabinoside. The effect of cytotoxic drugs upon the mitochondrial membrane potential may be relevance for the understanding of the mechanism of action, exerted by cytotoxic drugs upon cell biology.