Biocompatible inorganic-organic composite material for medical application as bone precursor is prepared through aggregation in aqueous suspensions of synthetic calcium phosphate and Acetobacter xylinum cellulose. The mineral component consists of two crystalline phases, hydrohyapatite Ña5(PO4)3(OH) and whitlocite Ña2.6Mg0.4(PO4) which are nanosized platelike crystals. The crystallites of hydrohyapatite and whitlokite are adsorbed on the surfaces of nanofibrilar cellulose ribbons. Whitlokite nanocrystals are predominantly deposited on them. Computer simulations of the structure of the inter-phase regions demonstrate that the formation of the hydrogen bonds between the surfaces of the calcium phosphates and cellulose gives the main contribution to the energy of the interaction of the mineral and organic components The whitlocite–cellulose interaction is more preferable in energy.