Mineralization of hydrogel biomaterials is considered desirable to improve their suitability as materials for bone regeneration. The most common strategy is the introduction of inorganic particles during hydrogel formation. In this study, gellan gum (GG) hydrogels were formed by simple mixing GG solution with bioglass microparticles, leading to hydrogel formation by ion release from the amorphous bioglass microparticles. This resulted in novel injectable, self-gelling composites of GG hydrogels containing 20% bioglass. Gelation occurred within an acceptable time frame for injection (within 20 minutes). Composites containing the standard 45S5 bioglass preparation were markedly less stiff. Synchotron X-ray µCT proved to be a highly sensitive and useful technique capable of detecting microparticles of diameter approximately 8 µm, i.e. individual microparticles. The size distribution of bioglass microparticles and their aggregates, and their distribution in GG hydrogels, was visualized accurately using this high-resolution technique. The widely used melt-derived 45S5 preparation was compared to a calcium-rich, sol-gel derived preparation (A2), as well as A2 enriched with zinc (A2Zn) and strontium (A2Sr). A2, A2Zn and A2Sr bioglass preparations were more homogeneously dispersed in GG hydrogels than the standard 45S5 preparation. Composites containing all four bioglass preparations exhibited antibacterial activity against methicilin-resistant Staphylococcus aureus (MRSA). Composites containing, A2Zn and A2Sr bioglass preparations supported the adhesion and growth of osteoblast-like cells and were considerably more cytocompatible than the standard 45S5 bioglass preparation. All composites underwent mineralization with CDHA upon incubation in SBF. The extent of mineralization appeared to be greatest for composites containing A2Zn and 45S5. The results underline the importance of the choice of bioglass when preparing injectable, self-gelling composites.