Neutron activation analysis is a physical tool by which various elements may be quantitatively determined, often with an extreme degree of sensitivity. Hitherto, factors of cost and distance involved in securing a neutron source have apparently precluded extensive biologic usage but, with the more recent availability of smaller nuclear reactors, more extensive biologic investigation may be predicted. The basic principles of neutron activation analysis might be briefly elucidated (1). A specimen, when placed in a reactor, is subjected to bombardment by thermal neutrons. The following scheme illustrates the resultant radiative capture (n, gamma) reaction: As an example, , an unstable isotope, then decays according to its usual scheme and, by virtue of its disintegrations, may be detected and quantitated. Thus the method is a twofold procedure; first, one induces radioactivity in a specimen by exposure to a neutron flux, and then the induced radioactivity is analyzed by the application of counting procedures...