The design is described of several liquid organic scintillation counters which have been used to detect 100‐Mev neutrons and gamma rays with the exclusion of lower‐energy particles and quanta.The neutron counter efficiency is a few percent and is nondirectional, properties not possessed by a more conventional counter telescope system. The gamma‐ray counter has an efficiency of about 50%, with a line width of about 10%.Particular problems arise in the use of these counters; counting rate stability is difficult to achieve; the maximum counting rate is limited by the pileup of individual pulses due to their finite width. These problems are discussed in detail and some solutions mentioned.