The use of an optical biosensor for rapid bioproduct analysis is described. The biosensor, which is sensitive to changes in the concentration of bioproduct at its biologically active surface, has been shown to provide concentration data within 10 s of sample addition to the device. This has been achieved through the use of linear regression analysis to extract information from the early part of the biosensor interaction profiles. The system has been used to monitor both the production and purification of antibody fragments expressed during batch fermentation of recombinant Escherichia coli. Data obtained using the biosensor have been used to provide real-time profiles describing the location of antibody fragments during bioprocessing. Biosensor data have also been compared with those obtained from ELISA, the traditional method of retrospective analyses of samples collected during bioprocessing.