Potassium fluorrichterite (KNaCaMg(5)Si(8)O(22)F(2)) glass-ceramics were modified by either increasing the concentration of calcium in the glass (GC5), or by the addition of P(2)O(5) to produce potassium fluorrichterite-fluorapatite (GP2). The solubility of the stoichiometric composition (GST), GC5 and GP2 were measured using the standard test described in ISO 6872:1995 (Dental Ceramics). Ion release profiles were determined for Si, Ca, Mg, Na, K and P using inductively coupled plasma mass spectrometry and fluoride ion (F(-)) concentration was measured using an ion-selective electrode. The cytotoxicity of all compositions was assessed using cultured rat osteosarcoma cells (ROS, 17/2.8). Cell response was qualitatively assessed using scanning electron microscopy (SEM) and quantitatively using the Alamar blue assay. GST was the least soluble and also released the lowest concentration of ions following immersion in water. Of the modified compositions, GC5 demonstrated intermediate solubility but the greatest ion release while GP2 exhibited the highest solubility. This was most likely due to GC5 having the greatest proportion of residual glass following crystallisation. The mass loss exhibited by GP2 may have been due in part to the partial disintegration of the surface of specimens during solubility testing. SEM demonstrated that all compositions supported the growth of healthy ROS cells on their surfaces, and this data was further supported by the quantitative Alamar blue assay.