Gamma-ray spectral analyses with a 5-cm × 5-cm LaBr3(Ce) detector and a NaI(Tl) detector of the same size show that the LaBr3(Ce) has much better gamma-ray peak resolution and full-energy peak counting efficiency but worse detection sensitivity. The LaBr3(Ce) detector has relatively high intrinsic radiation background due to the naturally occurring 138La radioisotope in lanthanum. Although this 138La background is entirely below the energy of 1,500 keV, additional background is in the energy region between 1,500 keV and 2,750 keV. The manufacturer attributes this radiation to alpha particles emitted by the five short-lived progeny of an 227Ac impurity. Comparative values for peak resolution, full-energy peak counting efficiency, and detection sensitivity are reported for 241Am, 60Co, and 137Cs. Results of counting 137Cs sources at two activity levels demonstrate the impact of background on detection sensitivity.