C129/U1 is a respiratory defective mutant of Saccharomyces cerevisiae arrested in cytochrome oxidase assembly due to a mutation in COX17, a nuclear gene encoding a low molecular weight cytoplasmic protein proposed to function in mitochondrial copper recruitment. In the present study we show that the respiratory defect of C129/U1 is rescuable by two multicopy suppressors, SCO1 and SCO2. SCO1 was earlier reported to code for a mitochondrial inner membrane protein with an essential function in cytochrome oxidase assembly (Buchwald, P., Krummeck, G., and Rodel, G. (1991) Mol. Gen. Genet. 229, 413-420). SCO2 is a homologue of SCO1, whose product is also localized in the mitochondrial membrane but is not required for respiration. SCO1 also suppresses a cox17 null mutant, indicating that overexpression of Sco1p can compensate for the absence of Cox17p. In contrast, neither copper, COX17 on a multicopy plasmid, or a combination of the two is able to restore respiration in sco1 mutants. Rescue of cox17 mutants by Sco1p suggests that this mitochondrial protein plays a role either in mitochondrial copper transport or insertion of copper into the active site of cytochrome oxidase. Although SCO2 can also partially restore respiratory growth in the cox17 null mutant, rescue in this case requires addition of copper to the growth medium. SCO2 does not suppress a sco1 null mutant, although it is able to partially rescue a sco1 point mutant. We interpret the ability of SCO2 to restore respiration in cox17, but not in sco1 mutants, to indicate that Sco1p and Sco2p have overlapping but not identical functions.