Based upon the CaO-P2O5 glass system, two glass ceramics were prepared in the meta-, pyro- and orthophosphate regions. The present work describes results concerning the biological activity of MT13B (45CaO-37P2O5-5MgO-13TiO2, in mol %) and MK5B (45CaO-45P2O5-5MgO-5K2O, in mol %) using MG63 osteoblast-like cells cultured for 6 days. The influence of a 24h pretreatment with culture medium before cell culture was also evaluated (MT13Bi; MK5Bi). Both “as received” and pre-treated material samples supported cell growth, which increased with the culture time. Pre-treatment resulted in an increase of adhered cells on both materials (day 1). Throughout the culture time (days 3 and 6), MT13Bi presented higher cell numbers than that of MT13B, but no differences were found between MK5Bi and MK5B. In addition, cell growth was significantly lower in the K-containing glass ceramic in both experimental situations comparing with Ticontaining glass ceramic. SEM and confocal microscopy observation showed that cell adhesion and spreading were hampered on MK5B samples. Evident macroscopic and microscopic surface instability was observed on this material with simultaneous dissolution/precipitation processes. By contrast, MT13B presented a relatively stable surface throughout the culture time. Results suggest that the pre-incubation with culture medium improves the adhesion of the osteoblast cells to both glass ceramics and that the high degradation rate of MK5B hinders the in vitro biological performance of this material.