Superoxide dismutase comprises a family of metalloenzymes that catalyze the oxido-reduction of superoxide anion to H2O2. Manganese superoxide dismutase (Mn-SOD) is encoded by nuclear chromatin, synthesized in the cytosol, and imported posttranslationally into the mitochondrial matrix. We isolated and sequenced complementary DNA encoding human Mn-SOD. The Mn-SOD cDNA was 1001 base pairs long with a single open reading frame. It contained 95 base pairs of 5' untranslated sequence, and 216 base pairs of 3' untranslated sequence, followed by a short polyadenylation tract. The deduced amino acid sequence suggests a mature protein of 198 amino acids preceded by a 24 amino acid leader peptide. A major transcript of 1000 nucleotides was identified by hybridization of the cDNA with RNA isolated from human cells. Precursor Mn-SOD was produced by in vitro transcription of the human Mn-SOD cDNA followed by in vitro translation utilizing rabbit reticulocyte lysate. The primary translation product of the cDNA is a polypeptide of Mr 26,000 as determined by sodium dodecyl sulfate-polyacrylamide electrophoresis. When the Mr 26,000 propeptide was incubated with freshly isolated rat liver mitochondria, the peptide was proteolytically processed to a Mr 24,000 polypeptide. Proteolytic processing was accompanied by an energy-dependent import of the peptide into the isolated liver mitochondria. Mature 125I-labelled Mn-SOD, isolated from rabbit liver, was not imported in vitro into mitochondria, indicating that the energy-dependent uptake of Mn-SOD by liver mitochondria was specific for the Mn-SOD precursor.