Although an unmineralized layer of organic material has been identified on both bone-forming surfaces and surfaces upon which bone formation has ceased (quiescent surfaces), the proportion of bone surfaces that is covered by unmineralized material has not been quantified. Because the unmineralized layer may play a role in the regulation of bone resorption, we undertook a scanning electron microscopy (SEM) assessment to determine its extent. Specimens of adult human ribs were prepared for undecalcified resin sections and SEM. For SEM, cells were removed and the bone surface was inspected and photographed. The same specimen was then immersed in NaOCl to remove organic material, and inspected again in the SEM. We found that the surface of bone appeared quite different before, compared to after, removal of organic material. Before removal, the entire nonresorptive surface was finely fibrillary. After removal of the organic material we observed a minor component showing the finely nodular surface typical of mineralizing bone, and a major component in which the mineral surface was free of such nodules. In only 3 of 1,200 photographs did we identify areas in which the bone surface was not altered by removal of organic material from the specimen. Analysis of histological sections of the ribs showed that approximately 85% of the bone surface was classifiable by light microscopy as quiescent. These results suggest that not only formative but also quiescent surfaces are covered by a layer of unmineralized organic material.