Osteopontin (OP), purified from rat bone, binds Ca2+ but whether different molecular forms of OPs derived from non-osteogenic sources and non-phosphorylated OP also possess this property remains to be determined. Furthermore, it is not known which specific site or sites of the molecule bind Ca2+ In the present-study, following an established procedure, total proteins in the conditioned media from OP-synthesizing cell cultures were separated by SDS-PAGE, transferred to Immobilon-P membranes, and incubated with 45CaCl2, then Ca2+ ions bound to protein bands were analyzed by autoradiography. Purified OPs, and synthetic oligopeptides representing specific domains of the OP molecule were adsorbed on the membrane and processed as described above. Our results show that OPs synthesized by normal rat kidney cells, oncogenically transformed Rat-1 cells, OP purified from human milk, and non-phosphorylated OP secreted by 1α,25-dihydroxyvitamin D3-treated mouse epidermal JB6 cells all bind detectable levels of Ca2+ with specificity. We also show that a synthetic peptide representing the domain of OP which contains nine consecutive aspartic acid residues binds Ca2+ with specificity. It is probable, therefore, that a Ca2+-binding site resides in this region of the OP molecule. We conclude that Ca2+-binding is a general property of OP, irrespective of its molecular mass and origin, and the phosphate moieties of OP may not influence the conformation or accessibility of the Ca2+ affinity sites of the molecule. © 1993 BY THE JOURNAL OF BIOCHEMISTRY.