Resistance to the antibiotics chloramphenicol (CAP^r) and efrapeptin (EF^r) in mammalian cells is cytoplasmically inherited^1,2. We demonstrate here that purified mitochondria obtained from CAP^r and EF^r cells are taken up by endocytosis and transfer antibiotic resistance to chloramphenicol-(CAP^s) and efrapeptin-(EF^s) sensitive cells. The resultant cells, termed mitochondrial transformants, are stably resistant to the antibiotics and are produced with high efficiency. The ability to transfer antibiotic resistance using mitochondria supports the notion that CAP^r is due to a mutation of the mitochondrial DNA³. This technique of mitochondrial-mediated transfer of antibiotic resistance may provide a novel means for studying mitochondrial genetics in mammalian cells.