G–M counting assemblies have been constructed which give dependable low background operation in routine use. Possible construction materials have been examined in detail for radioactive content in an effort to achieve minimum inherent activity in the assembly. The cosmic ray component of the normal background has been eliminated by an anticoincidence arrangement. Increased reliability of the system is obtained through (a) use of a multiple anode anti-coincidence counter in place of a bundle of tubes, (b) use of an electronic quench on both sample and shielding counters, and (c) operation of the assembly as flow counters in tandem. In the best arrangement to date a copper counter with a polystyrene seal gives a background of 0.9 counts per minute.