A highly collimated mobile gamma-ray spectrometer constructed by EG&G Idaho has been calibrated for full-energy peak efficiency. This spectrometer contains a high resolution intrinsic germanium detector, and it is designed for making activity measurements inside reactor containment buildings in high radiation fields. Peak efficiencies were determined for point sources positioned on and off the detector-collimator axis over an energy range from 80 to 2800 keV. The off-axis distributions were integrated to give the isotropic disk source efficiencies. These results were compared to those obtained from directly measuring a 24Na-doped aluminum foil “disk.” Theoretical disk source efficiencies were calculated using the point kernel approach. These results were normalized to the on-axis point source efficiencies. The procedure is self-consistent but it requires more detailed measurements in the future in order to provide accurate results. Once the measurements are made, however, the spectrometer system can then be used to accurately assay arbitrary distributions of surface activities. The method is also well suited for use in collimator design because leakage effects can be directly calculated.