[en] Instrumental neutron activation analysis has been used to determine the concentrations of 32 elements in three reference materials, for an intercomparison organized by the Institute of Radioecology and Applied Nuclear Techniques, Koshice, Czechoslovakia, with the participation of 34 laboratories from 11 countries. Fly-ash materials from coal fired power plants were analyzed. The concentration of Ba, Ca, Ce, Co, Cr, Cs, Eu, Fe, Hf, La, Lu, Nd, Rb, Sb, Sc, Sm, Sr, Ta, Tb, Th, U, Yb, Zn, Zr was determined after a long irradiation (50 hours) of the samples in a thermal neutron flux of 1,1x10¹¹ n/cm².s. The samples and Soil-5, SL-1, GSP-1 as standards were measured 2-5 hours after decay time of 10-30 days. The results obtained for ENO, EOP and ECH are in generally good agreement with the certified values