Mitochondria in cardiac muscle cells and myoblast-fused myotubes display unusually long (3-5 days) retention times of rhodamine 123, a mitochondria-specific fluorescent probe, in living cells. Among 50 keratin-positive carcinoma or transformed epithelial cell lines tested, mitochondria with prolonged rhodamine 123 retention are detected in most of the transitional cell carcinoma, adenocarcinoma, and chemical carcinogen-transformed epithelial cell lines and in some squamous cell carcinoma lines but not in any oat cell carcinoma lines. The presence of mitochondria having unusual dye retention may be useful for diagnosis and exploitable for chemotherapy of certain human carcinomas.