Although overproduction of recombinant proteins by mammalian cells is well established, little attention has been paid to analysis of the quality of the products. We focused on the quality of antibodies produced during the death phase of a recombinant Chinese hamster ovary (CHO) cell line. The quality of the monoclonal antibody against HM1.24 antigen and the post-translational characteristics of the subunits during CHO cell culture in a 160-L bioreactor were investigated. The culture supernatant of a stable cell line was collected and purified by affinity chromatography and then analyzed. There were no significant changes in gel-permeation chromatography variables, carbohydrate structure, or antibody-dependent cellular cytotoxicity activity during the death phase of cell culture. However, ion-exchange chromatography analysis revealed that antibody heterogeneity changed, as indicated by a decrease in cell viability. The results presented here provide useful information that will help in determining the time to end each batch culture. (C) 2009, The Society for Biotechnology, Japan. All rights reserved.