The hydrophobic fluorescence dyes NAO and DPPAO (see scheme of structural formulae) stain the mitochondria of living HeLa-cells. The trans-membrane potential favours the dye accumulation of the cation NAO and supports the hydrophobic interaction of the dye with the mitochondrial membrane lipids and proteins. The lecithin-like dye DPPAO is electrical neutral. Its binding to mitochondria of living cells is only caused by hydrophobic interaction. NAO and DPPAO stain also the mitochondria of glutaraldehyde fixed HeLa-cells in aqueous medium. Fluorescence staining occurs even after extraction of the lipids of the cell with acetone. We suppose that the dye accumulation in the mitochondria of the fixed cells is caused by the hydrophobic interaction between the dyes and the very hydrophobic mitochondrial lipids and proteins.