Prolidase (peptidase D) catalyzes hydrolysis of the di- and tripeptide with carboxyl-terminal proline and plays an important role in recycling proline in various cells and tissues. By using human prolidase cDNA as a probe, a chromosomal gene related to prolidase was isolated from human gene libraries. The human prolidase gene is over 130 kilobases long and is split into 15 exons. All of the splice donor and acceptor sites conform to the GT/AG rule. The transcription initiation site was determined by nuclease S1 mapping and primer extension and was located 131 bases upstream from the initiation codon. A "CAAT" box-like sequence was present 67 bases upstream from the cap site, but there was no "TATA" box-like sequence. There were seven sets of sequences resembling the transcription factor Sp1 binding sites. Four were upstream from the cap site, and three were downstream. We also analyzed findings in patients with prolidase deficiency with respect to major gene re-arrangement. Several hundred base deletions, including the 14th exon, were identified. Knowledge of the gene structure of human prolidase will facilitate further studies on the expression and regulation of this gene and provide necessary information for analyses of mutations in patients with this deficiency.