In response to a question by Oesch, Uehleke estimated that about 50–60% of the covalently bound CCl4 is metabolized in mitochondria. Oesch referred to the concept that the free radicals which are often thought of as intermediates may not be stable enough to survive translocation from the endoplasmic reticulum (ER) to mitochondria. He reported that there is evidence that ER can form complexes with mitochondria and suggested that the complexes play a role in heme protein synthesis, because inducing conditions increase the number of complexes. Thus it may be possible that the reactive species need not leave the lipid matrix of the ER. He inquired about the relative half-life of the radicals, and about the possibility that part of the metabolism would occur in mito?chondria. Uehleke felt that the compounds are not metabolized by purified mitochondrial preparations, as these contain only about 0.1% P450. He agreed that there is electron microscopic evidence of tubular structures of the ER penetrating mitochondria. He pointed out that the half-life of the free radical in the gas phase is only one- to two-thousandths of a second. However, Steers (Göttingen) has obtained good evidence that radicals derived from aromatic amines, and possibly also the trichloromethyl radical, are stable in the lipid layer for minutes, some even for hours.