Following interaction with simulated body fluid, apatite and calcite were identified on the surface of bioactive glasses. The phase composition of the glass surface layer after elution is affected by the experimental conditions. At a high ratio of sample surface area (S) to the volume of the leaching solution (V), calcite was identified in the surface layer, whereas a low ratio (a large volume of the solution) had led to the formation of apatite. From the point of view of crystallography, the apatite was not well developed, and its detection limit by the X-ray diffraction method was less than 5%. The calcite was well developed. Phosphorus-free glasses exhibited a higher tendency to calcite formation than the phosphorus containing ones. Suitable experimental conditions with respect to the S/V ratio have to be chosen in the "in vitro" testing to ensure availability of phosphorus in the solution throughout the exposure, as otherwise calcite would eventually precipitate on a thin layer of apatite.
