The ultrastructural localization of osteopontin in bone was determined especially focussing on the relationship to bone forming cells, i.e. osteoblasts and osteocytes. Thus, rat metaphyseal and cortical bone was fixed in a mixture of low concentration glutar- and paraformaldehyde and embedded at low temperature in Lowicryl K11M. Polyclonal antibodies raised against rat osteopontin fusion protein were incubated on ultrathin sections and protein G coated with 5-nm colloidal gold was used for detection. The results demonstrate most intensive labeling in the mineralization front of newly formed bone; whereas lower concentration of label was found in the osteoid both in metaphyseal and cortical bone. The concentration of marker was substantially higher in newly formed bone near osteoblasts compared to bone constituting the osteocyte lacuna. Intracellularly the prevailing localization of label was to large Golgi vesicles in osteoblasts. Only focally local accumulation of marker was seen at the cell/osteoid surface. The observations suggest a function of osteopontin also in the mineral turnover of newly formed bone.