Monoclonal antibody (mAb) fragmentation can be a widespread problem across the biotechnology industry and there is a current need to better understand the underlying principles. Here, we report an example of a high-purity human IgG1 mAb prepared from CHO cells exhibiting fragmentation that can be attributed to residual proteolytic enzyme activity. The concomitant occurrence of proteolytic and non-proteolytic peptide bond cleavage is shown and the respective fragmentation patterns characterized using high-resolution LC-MS. Fragmentation rates are monitored by SE-HPLC and SDS-PAGE over the pH range 4-6 and characterized in the presence and absence of pepstatin A, an inhibitor of acidic proteases. After 20 days at 40 degrees C, pH 4, similar to 60% decrease in BIIB-mAb monomer peak occurred attributed to residual proteolytic activity. At pH 5, this value was similar to 13%. These results have implications for formulation design studies and the interpretation of accelerated stability data. A simple method to screen for acidic protease activity using the proteolytic enzyme inhibitor pepstatin A is described. Biotechnol. Bioeng. 2011;108: 977-982. (C) 2010 Wiley Periodicals, Inc.