Neutron activation analysis, employing low flux intensities ( 2 s ̇ 5 × 10 8 neutrons/cm 2/ sec ) has been adapted to the estimation of anions and cations in biologic specimens. A flux of this magnitude is obtainable from Van de Graaff-beryllium sources, radium-beryllium systems, as well as nuclear reactors; hence the techniques should be technically feasible for a number of laboratories. Following exposure of 10 ml serum to such a flux for one hour, 15 radioisotopes are produced to the extent of 30 dpm or greater. The isotopes represent 8 cations (Na, K, Ca, Cu, Zn, Mg, Co, Mn) and 4 anions (Cl, Br, I, P). Methods of separating the isotopes, and distinguishing them by beta and gamma ray spectroscopy are discussed. Radiosodium (Na 24) is the principal gamma ray emitting isotope produced in blood; it may be readily produced and estimated with fluxes of 10 6 or even lower. The necessity of removing radiosodium in order to estimate isotopes present in only trace quantities is stressed. Applications to the nondestructive analysis of tissues, red cells, and other biologic specimens is discussed.