3H-labeled chemically modified, non-antimicrobial tetracycline (4-de-dimethyl-aminotetracycline; CMT) was injected intraperitoneally into neonatal rats and the animals killed at 20 min, 2 or 4 h after isotope injection. The binding of 3H-CMT in the calvariae was localized by light and electron microscopic autoradiography. At 20 min after injection, a high level of bound 3H-CMT was localized in the cells of the periosteum including the osteoblasts and preosteoblasts. The newly formed bone matrix was also heavily labeled by silver grains. At the ultrastructural level, many silver grains appeared over the cytoplasm of osteoblasts and preosteoblasts, particularly along the plasma membranes, in the periosteum. At 2 and/or 4 h after isotope injection, the number of silver grains over the cells was markedly reduced by that over the bone matrix appeared unchanged. These results indicate that, in addition to the mineral phase of bone, CMT directly binds to (or is taken up by) osteogenic cells and is then rapidly metabolized by these cells.