Biocompatible and resorbable melt derived glass compositions which include: SiO2 60-70 weight-%, Na2O 5-20 weight-%, CaO 5-25 weight-%, MgO 0-10 weight-%, P2O5 0.5-3.0 weight-%, B2O3 0-15 weight-%, Al2O3 0-5 weight-%, and which contain less than 0.05 weight-% potassium. Biocompatible and resorbable glass fibers manufactured from these glass compositions, medical devices containing fibers of the invention, the use of these compositions for the manufacture of glass fiber and the use of the fibers for the manufacture of medical devices are also disclosed.