In the studies undoped HA and HA modified with 0.3; 0.6; 0.9; 1.8 wt % of Mg were prepared by the wet method. Introduction of magnesium into HA structure influenced its thermal stability as well as phase composition, sinterability, microstructure, flexural strength and chemical stability of the obtained calcium-phosphate ceramics. The presence of magnesium promoted the decomposition of HA to βTCP above 800°C. Beyond a certain limit (0.9 wt %), Mg ions caused formation of MgO in Mg-HA ceramics. Chemical stability of Mg modified HA below 0.9 wt % Mg under in vitro conditions was similar to that of the undoped hydroxyapatite. Biological studies showed that the number of cells cultured on the surface of HA samples with 1.8 wt % Mg additive, probably due to the MgO content, was lower than on the pure HA ceramics.