The present invention discloses an on-line system, a method for its calibration and simultaneous detection of antibiotic residues and their concentrations in milk. The system comprises a device for lactose hydrolysis to increase the concentrations of glucose and galactose in milk samples at least 10 times to obtain their equal concentrations in all investigated samples and a biosensor array to measure the transient phase decrease of dissolved oxygen concentration caused by the enzymatic oxidation of glucose and galactose. Parameters, characterising the oxidation of glucose and galactose, are calculated for each biosensor on the basis of the transient phase decrease of dissolved oxygen concentration and initial oxygen concentration in milk sample. The combination of different parameters of different biosensors forms a pattern of milk sample and in the presence of antibiotics this combination forms the fingerprints of particular antibiotics.
