The aggregation process and conformational change of two monoclonal antibodies, monitored by in situ measuring techniques during stressing in a well-defined flow field, was experimentally studied. The increase in turbidity, which was detected by in-situ UV-vis spectroscopy, was attributed to the formation of aggregates. A stronger aggregate formation could be related to the higher exposure of hydrophobic groups, which was monitored by in situ fluorescence spectroscopy. Both presented techniques are powerful methods to monitor the aggregation process of monoclonal antibodies directly in flow fields.