When completely demineralized, the densely packed structure of bone matrix does not recalcify, neither in physiologic solutions in vitro nor in implants in vivo. Even when inorganic and organic calcification inhibitors (which normally are stored in bone matrix) are removed first by autolytic digestion in neutral buffers at 37C and then by sequential chemical extraction, implants of the EDTA insoluble residue will not recalcify after as long as 4 wk in a muscle pouch. However, if first demineralized in cold dilute HCl, second, extracted and autodigested in buffers solution at 37C, and then further extracted in EDTA and other solutions at 2C, a calcification initiator protein (Cp) is unmasked, and the residue will invariable recalcify. CIP, isolated by gel filtration and column chromatography, is a disulfide-bonded glycoprotein aggregate composed of subunites of a moleclar mass of 55,000. CIP is composed of a large proportion of acidic amino acids and has a calcium binding capacity of about 1.8 times greater than albumin. The affinity constant CaCIP, calculated by ultrafiltration of physiologic solutions of Ca2+, is log K, 2.9. Observations on implants of residues that containe a) CIP but not a bone morphogenetic property (BMP), B) BMP accompanied by CIP activity, or c) neither BMP nor CIP activity suggested that BMP covers CIP and that the two are attached to bone collagen in tandem. Whether CIP plays a part in calcification of the normal skeleton requires further investigation.