E. coli SlyD, a prolyl isomerase, specifically binds divalent metal ions, which can result in significant contamination of IMAC preparations of heterologously expressed His-tagged proteins. A side-by-side comparison of BD TALON™ Resin (a Co2+-based resin) and Ni-NTA was performed to analyze SlyD binding. Ni-NTA bound substantial quantities of SlyD under both native and denaturing conditions whereas no SlyD binding was detected with BD TALON Resin. Because BD TALON resin is less prone to SlyD contamination than Ni-NTA resins, it is the IMAC resin of choice, particularly when purifying poorly expressed proteins.