An orally bioavailable aqueous solution containing the chloride salt of the radioisotope calcium-41 (41Ca) with phosphate-binding and radioisotopic activities. Upon administration of calcium-41 chloride aqueous solution, calcium-41 is preferentially taken up by osteoblasts, which generate mineralized osteoid containing calcium. Calcium-41 accumulation and turnover in bone can be measured with bone scintigraphy and urinary isotope excretion.