An optimum procedure is outlined for the rapid quantitative determination of trace (>0.06 ,ug) selenium in serum and tissue using 77mSe neutron activation. Sample preparation, irradiation procedures, types of detector assemblies, and spectral analyses and data reduction are discussed. The optimum procedure utilizing a low-power nuclear reactor (~3 X 1011 n/cm2 sec) consisted of irradiating lyophilized tissue samples or dialyzed-lyophilized serum