The canine renal distal tubular cell line Madin-Darby canine kidney (MDCK) forms calcium phosphate microliths during a long-term culture in vitro. We identified osteopontin (OPN) and calprotectin (CPT) from a urinary stone matrix. We recently also detected the expression of OPN and CPT in MDCK cells. The relationship between the mechanism of the stone formation and these stone matrix proteins is not yet known. Here, MDCK cells were cultured and inoculated in the subcutis of nude mice. After 4, 8 and 12 weeks, the inoculated tissues were resected, fixed and immunostained with polyclonal anti-human OPN and polyclonal anti-human CPT antibodies. Some serial specimens were stained with von Kossa's procedure. MDCK cells formed some follicular formations in the subcutis of nude mice at least at 12 weeks after transplantation. At 8 weeks after the inoculation, we detected small calcium phosphate stones with MDCK cells trapped in the follicles. The cells forming the stones also expressed both OPN and CPT. The CPT expression sites coincided with the stone formation sites. We confirmed that MDCK cells inoculated in nude mice had stone-forming potential, and we speculate that OPN and CPT play important roles in stone formation by MDCK cells.