Prolidase deficiency is an autosomal recessive disorder characterized by mental retardation and various skin lesions. Cultured skin fibroblasts were obtained from two independent patients with abnormal prolidase. Using the polymerase chain reaction, we amplified the entire coding region of human prolidase mRNA derived from patients' fibroblasts. Nucleotide sequence analysis of amplified cDNA products revealed a G to A substitution at position 826 in exon 12, where aspartic acid was replaced by asparagine at the amino acid residue 276, in cells from both patients. An analysis of the DNA showed that the substitution was homozygous. An expression plasmid clone containing a normal human prolidase cDNA (pEPD-W) or mutant prolidase cDNA (pEPD-M) was prepared, transfected, and tested for expression in NIH 3T3 cells. Incorporation of pEPD-W and pEPD-M resulted in the synthesis of an immunological polypeptide that corresponded to human prolidase. Active human enzyme was detected in cells transfected with pEPD-W, but not in those transfected with pEPD-M. These results were compatible with our observation of fibroblasts and confirmed that the substitution was responsible for the enzyme deficiency. As active prolidase was recovered in prolidase-deficient fibroblasts transfected with pEPD-W, this restoration of prolidase activity after transfection means that gene replacement therapy for individuals with this human disorder can be given due consideration.