Glasses based on the Bioglass, formula and doped by the addn. of CeO2 (1.5-13.5%) and ZnO (5-20%) were studied. The addn. if small oxide quantities (1.5% for CeO2 and 5-10% for ZnO) did not significantly alter the ability of the in vitro apatite formation on the glass surface. After 15 days, a mixed-metal (Ca-Zn) or (Ca-Ce) phosphate-based layer was identified. High metal contents improved the chem. durability of the glass, the bioactivity was strongly inhibited and there were no hints of apatite formation obsd. even after prolonged soaking in SBF. The SiO2, 39.4; Na2O, 22.6; CaO, 22.6; P2O5, 10.4; ZnO, 5.0% glass compn. seems to demonstrate an optimal ratio of Zn/P to improve the glass strength and simultaneously yield a Ca/P ratio able to preserve rapid apatite formation.