Soluble organic matrix was extracted with 0.1 M EDTA from calcium oxalate monohydrate urinary stone (CaOx), purified by SDS-polyacrylamide gel electrophoresis and transferred to polyvinylidene difluoride membranes for the NH2-terminal amino acid sequence analysis. A protein at 30 kDa showed complete homology with calprotectin (20 amino acids in the NH2-terminal), so we termed this a calprotectin-like protein. Calprotectin which was extracted from human granulocytes had inhibitory activity in calcium oxalate monohydrate crystal growth in vitro. This study suggested that, calprotectin-like protein detected in CaOx is thought to be a potent inhibitor of crystal growth and may therefore be important in the etiology of CaOx formation.