The accurate measurement of slow neutrons for biological studies is a difficult and important problem. Measurement is complicated because of the nature of the methods required for the production of an adequate neutron flux for biological investigations. It is further complicated because there is no method at present for the direct determination of tissue dose. However, use of indirect methods developed in the past few years will enable the radiobiologist to measure thermal neutrons in terms of relative biological effect with reasonable accuracy. In order to achieve adequate numbers of slow neutrons for biological work it is necessary to use fast neutrons as the primary source. For acute studies, and where relatively large fluxes are needed, nuclear reactors are best. For chronic studies, polonium-beryllium and radium-berryllium sources may be useful. No matter what the originating reaction, the complications of measurement are similar. For instance, slow neutrons from a nuclear reactor are produced as follows. Fission occurs in a critical assembly, and both fission neutrons and gamma rays are produced. The fission neutrons have an energy spectrum extending from the thermal region to above 10 Mev, with the maximum of the energy distribution curve occurring at about 700 kev (1). Only a small portion of all the neutrons produced are "slow." The fission neutrons are degraded in energy by the use of a suitable moderator, such as graphite or heavy water, to produce slow neutrons to continue the chain reaction. Those not used to maintain the chain reaction are available for experimental use. However, not all the excess energy from degradation of the neutrons appears as heat or vibrational energy of the struck atoms of moderator. A portion may appear as gamma rays produced by inelastic scattering. Also, capture in, and activation of, the moderator material or its impurities will produce gamma photons. The above method provides a good slow-neutron source, but the slow neutrons are accompanied by coexisting radiations: fast neutrons, fission gamma rays, and gamma rays characteristic of the production method and the nature of the moderator and its purity. Although the above example concerns the use of the nuclear reactor as a slowneutron source, the measurement of neutrons from polonium-beryllium and radiumberyllium sources is complicated also by coexisting radiations. For physical experimentation with slow neutrons, the coexisting radiations may