The induction of antibiotic-resistant mutations in yeast mitochondrial DNA by manganese is decreased when the manganese-containing medium is additionally supplemented with magnesium. At equimolar concentrations of manganese and magnesium the former is no longer mutagenic. Amino acid starvation, cycloheximide, chloramphenicol and erythromycin have very little, if any, effect on the mutagenicity of manganese. Hydroxyurea itself seems to be slightly mutagenicity of manganese. Our results show that manganese acts as an error-producing factor in DNA replication probably through a direct interaction with mitDNA polymerase(s).