The planar and single photon emission computed tomography (SPECT) imaging performance of two low-energy and one medium-energy collimators has been compared for 123I(p,5n). Septal penetration in the low-energy collimators affected the planar uniformity and sensitivity and made the usual uniformity correction methods inappropriate. SPECT uniformity and resolution were also distorted by these artifacts induced in the planar images obtained with the low-energy collimators. The planar and SPECT images of a SPECT phantom contradicted the spatial resolution measurements made with a line source in air. The medium-energy collimator is recommended for imaging 123I(p,5n)-labeled antibodies in patients, particularly when quantitation is required.