Accurate protein identification sometimes requires careful discrimination between closely related protein isoforms that may differ by as little as a single amino acid substitution or post-translational modification. The ABRF Proteomics Research Group sent a mixture of three picomoles each of three closely related proteins to laboratories who requested it in the form of intact proteins, and participating laboratories were asked to identify the proteins and report their results. The primary goal of the ABRF-PRG04 Study was to give participating laboratories a chance to evaluate their capabilities and practices with regards to sample fractionation (1D- or 2D-PAGE, HPLC, or none), protein digestion methods (in-solution, in-gel, enzyme choice), and approaches to protein identification (instrumentation, use of software, and/or manual techniques to facilitate interpretation), as well as determination of amino acid or post-translational modifications. Of the 42 laboratories that responded, 8 (19%) correctly identified all three isoforms and N-terminal acetylation of each, 16 (38%) labs correctly identified two isoforms, 9 (21%) correctly identified two isoforms but also made at least one incorrect identification, and 9 (21%) made no correct protein identifications. All but one lab used mass spectrometry, and data submitted enabled a comparison of strategies and methods used.