The puromycin-sensitive aminopeptidase (PSA) is thought to contribute to the degradation of enkephalins. Besides being the most abundant aminopeptidase in the brain, PSA is expressed in other organs as well. From a human fetal brain cDNA library, we have isolated a cDNA encoding the human PSA (huPSA) protein. The isolated cDNA gave rise to a protein with a molecular mass of 99 kDa. Compared with mouse PSA, homology at the amino acid and cDNA level was 98 and 93%, respectively. Translation of the huPSA was found to be initiated at the second of two possible start codons, as shown by studies with antibodies directed against peptide sequences of both potential N-terminal regions. Northern blot analysis with RNA isolated from different human organs demonstrated that the huPSA transcript is strongest but not exclusively expressed in the brain. Vesicular stomatitis virus epitope-tagged huPSA protein was expressed in HeLa cells and found to be localized in the cytoplasm, especially in the perinuclear region. By in situ hybridization, huPSA transcript could be identified in cortical and cerebellar neurons, whereas glial cells and blood vessels remained negative.