Glass encapsulated LiF microdosemeters were used to investigate the X- and gamma -ray dose delivered to the lungs of dogs by a lung scanning agent used in nuclear medicine. The microdosemeters were injected into the jugular veins of the dogs immediately after the ⁹⁹Tc^m aggregates had been injected. They were small enough to pass through the heart but became lodged in small lung arteries. The dogs were sacrificed one day after the injection so that the dosemeters could be recovered and read out. A value of 189+or-16 mrad mCi^-1 of lodged ⁹⁹Tc^m agent was measured as the average X- and gamma -ray dose to the dogs by this procedure. After subtracting the dose to the lungs due to fragmented agent lodged in the liver and spleen, the results were found to be in reasonable agreement with the X- and gamma -ray dose calculated from estimates of the absorbed fraction.