The amount of cytochrome b5 was not changed significantly by a single injection of cobalt (60 mg/kg body weight) or by daily injection of cobalt (30 mg/kg body weight) for 4 days or 8 days. On the other hand, the amount of cytochrome P-450 was depressed strongly by both cobalt treatments. 2. The incorporations of [3H]leucine as well as 5-amino[3H]levulinate into cytochrome b5 in cobalt-treated animals were almost the same as those in the controls 5 h after injections of the radioisotopes, whereas the radioactivity of heme labelled with 5-amino[3H]levulinate in microsomal residues after trypsin digestion, which would consist mainly of cytochrome P-450, was higher in the controls than in cobalt-treated animals after 5 h.
