Prolidase deficiency is a rare autosomal recessive disorder characterized by iminodipeptiduria, severe skin ulcers, recurrent infections, and mental retardation. The enzyme prolidase hydrolyzes dipeptides containing C-terminal proline or hydroxyproline. We investigated the metabolic abnormality caused by prolidase deficiency in human cultured skin fibroblasts. These studies were undertaken to test biochemical hypotheses regarding the metabolic origins of the skin lesion occurring in this disease. Our results indicate that prolidase plays a major role in the recycling of dipeptide-bound proline. Control fibroblasts were able to use iminodipeptides in lieu of proline to sustain normal growth, whereas cells homozygous for the prolidase deficiency mutation were not. Proline derived from iminodipeptides diluted incorporation of radiolabeled extracellular proline into cellular protein in normal cells but not in mutant cells. Substitution of a prolidase-free medium for FCS did not affect the growth rate of control cell lines but increased the doubling time of prolidase-deficient cells by 19% (28% in the presence of iminodipeptides). Iminodipeptides added to control and mutant cells maintained in serum-free medium showed no adverse effects on protein synthesis. These results are consistent with a mechanism of biochemical pathology in which proline deprivation caused by the enzyme deficit is a primary cause of damage to skin cells. Prolidase regulation by product and substrate was studied. A 44% decrease in activity was observed in fibroblasts grown for 3 wk in proline-containing medium relative to proline-free medium. However, cells grown in medium in which iminodipeptides replaced proline showed no significant difference in prolidase activity.