Abstract: Biosynthesis of heme A, a prosthetic group of cytochrome oxidase (COX), involves an initial farnesylation of heme B. The heme O product formed in this reaction is modified by hydroxylation of the methyl group at carbon C-8 of the porphyrin ring. This reaction was proposed to be catalyzed by Cox15p, ferredoxin, and ferredoxin reductase. Oxidation of the alcohol to the corresponding aldehyde yields heme A. In the present study we have assayed heme A and heme O in yeast COX mutants. The steady state concentrations of the two hemes in the different strains studied indicate that hydroxylation of heme O, catalyzed by Cox15p, is regulated either by a subunit or assembly intermediate of COX. The heme profiles of the mutants also suggest positive regulation of heme B farnesylation by the hydroxylated intermediate formed at the subsequent step or by Cox15p itself.