OBJECTIVE: To isolate, purify, sequence and characterize nephrocalcin (NC), a urinary protein that may be an important determinant of calcium oxalate (CaOx) kidney-stone disease. MATERIALS AND METHODS: Proteins were isolated from human urine using cellulose and resin columns and were sequenced using Edman degradation and SDS-polyacrylamide gel electrophoresis (SDS-PAGE). Inhibition of CaOx crystal growth by the isolated proteins was assessed by measuring the deposition of 14C-labelled CaOx. RESULTS: A protein assumed to be NC on the basis of SDS-PAGE, inhibitory and gel filtration properties was isolated from healthy human urine. Its molecular weight and the amino acid sequences of two of its peptides suggested it was identical to fragment HI-14 of the light chain (bikunin) of inter-alpha-trypsin inhibitor (ITI). CONCLUSIONS: NC represents a portion of the light chain of ITI, although this conclusion must remain tentative until confirmed using authentic NC.