Proteolytic activity of plasmin in milk, which leads to cleavage of casein, affects the quality of milk products (Crudden et al., 2005). Therefore currently, the substantial effort is focused on the development of new methods, that would allow inexpensive and rapid detection of plasmin in milk and milk products. We worked on development of a biosensor for the detection of plasmin concentrations based on quartz crystal microbalance (QCM). We tested two biosensor systems for plasmin detection. In the first system, we immobilized casein on a surface of QCM transducer, which served as a substrate for plasmin. The other one was based on monitoring of the interaction of plasmin with a layer of a plasmin-specific peptide, plasmin substrate (SP), immobilized on the gold electrode of the QCM. Keywords: plasmin, plasmin substrate (PS), quartz crystal microbalance (QCM), casein