Three fly ashes from wood biomass combustion were studied for d., unburned C content, particle size distribution, chem. compn. (as oxides), concn. of natural radionuclides (K-40, Ra-226 and Th-228) and heavy metals as well as changes in temp. relationship of resistivity. No heavy metals (Hg, Pb, Cd, Cr, Ni and Bi) were found in the fly ashes. The compn. of ashes and low concn. of radionuclides indicated the possibility of using them in construction and agriculture.