Certain elements of biological interest cannot be measured by conventional neutron activation analysis. Some elements lead, by (n,γ) reaction, to radionuclides too short-lived to be measured by their gamma activity or to stable elements. With other elements, such as sulfur and phosphorus, neutron activation produces pure beta emitters, which are difficult to measure without destruction of the sample. Samples of blood, bone, and hair were irradiated in vitro at the outlet of the curved neutron guide of the Saclay reactor EL3 in a flux of thermal neutrons (⟨ 0.127 eV). The capture spectra were recorded by means of a 20 cm3 Ge(Li) detector. The elements H, B, Cl, Na, K, N, S, and P were identified. In addition, boron, hydrogen, and chlorine were determined in two samples of cabbage and brown seaweed. Since a homogeneous irradiation was impossible because of the weak penetration of the thermal neutrons in the biological sample, it was necessary to use an internal standard (mercury). By the capture-gamma method of analysis it was also possible to measure in vivo the Ca/Cl mass ratio of a human tibia.