Osteonectin is a major glycoprotein of porcine and bovine bones and teeth that is found associated with hydroxylapatite crystal surfaces. From the ability of osteonectin to bind calcium ions, it has been proposed as a possible nucleator of hydroxylapatite crystal formation. Analysis of hydroxylapatite-bound proteins of rat bone and dentine, however, has revealed that osteonectin represents only 2.5 +/- 1.5% of the hydroxylapatite-bound protein in long bones, 0.9 +/- 0.5% in calvariae, and less than 0.1% in incisor dentine of animals of different ages. Further, in vivo pulse-chase studies carried out in young adult rats have shown osteonectin to be synthesized at low levels in these tissues. Similarly, low levels of osteonectin were synthesized by rat calvarial cells in vitro. In contrast, fibroblastic cells from periodontal ligament and gingiva synthesized significantly greater amounts of osteonectin. These studies indicate that the low quantities of osteonectin in rat mineralized tissues are a consequence of low rates of formation rather than being due to rapid turnover. The virtual absence of osteonectin in incisor dentine correlates with the lack of peritubular dentine in rat, whereas the low osteonectin content of rat bones may reflect differences in their structure and biophysical properties compared with bones of larger mammals.