Clinical, biochemical and genetic studies in a family with inherited, renal iminoglycinuria showed that the proband had a prominent renal tubular defect for reabsorption of glycine, proline and hydroxyproline. His parents and several relatives had hyperglycinuria only. No defect in intestinal absorption or transport of glycine or the imino acids was found in the proband. Familial iminoglycinuria is concluded to represent another specific inborn error of transport due to an autosomal mutation. Homozygotes for this mutation are believed to excrete abnormal quantities of the imino acids and glycine whereas heterozygotes excrete excessive glycine only. Glycine is reabsorbed by more than one renal transport system, and the biochemical mechanisms that control intestinal transport of glycine and the imino acids are not identical. The abnormal phenotype noted in iminoglycinuria is very probably due to more than a single abnormal genotype.