To ensure that the mAb-sCT conjugate was not cytotoxic to cells other than osteoclasts, we employed the MTT assay, in which mitochondrial dehydrogenases of viable cells cleave the tetrazolium ring of MTT, yielding purple formazan crystals. An increase or decrease in cell number results in a concomitant change in the amount of formazan formed, indicating the degree of cytotoxicity caused by the test material.