The counting efficiencies of two lung monitor systems—phoswich detector system and germanium detector system—were measured for lungs and liver loaded with 241Am in the JAERI phantom. It was found that the germanium detector system for 241Am loaded lungs counting gives the full-energy peak predominated by the absorption of the gamma-ray photon in a single photoelectric interaction and is less sensitive to 241Am loaded liver. The 241Am loaded lung activities could be reasonably estimated using germanium detector system rather than phoswich detector system.