An online, analytical technology was developed that utilized fluorescence to detect cells during an immobilized cell culture process. Chinese hamster ovary (CHO) cells that produced monoclonal antibodies (mAb) were transfected to express green fluorescent protein (GFP), and stable, fluorescence-positive cells were obtained by fluorescence-activated cell sorting (FACS). The immobilized cell culture process was then used to test the effects of sodium butyrate on cells. In this study, cells were cultured in porous, fibrous matrices that were placed in spinner flasks. A lab-scale, perfusion bioreactor with computer-controlled, online fluorescence sensors that continuously detected GFP fluorescence and quantified cell growth was utilized. In addition, the level of GFP fluorescence was used to predict mAb production in the culture without sampling for cell counting and protein analysis. Thus, non-invasive, fluorescence detection of cells provided a rapid, reliable and robust approach for developing an immobilized cell culture process. (C) 2011 Elsevier Ltd. All rights reserved.