The objective of the present research study was to develop a method to identify low-molecular weight fraction of bovine β-casein hydrolysis products by plasmin using a reversed-phase high performance liquid chromatography (RP-HPLC) method on-line with an ultraviolet (UV) spectroscopy method. The bovine β-casein was hydrolysed by the plasmin. The supernatant left over after the precipitation of hydrolysate was separated via ultra-filtration into a retentate containing polypeptides and a permeate containing low molecular weight peptides. To identify the fraction of permeate, those parameters were used, which quantitatively characterised the second and fourth derivatives of UV spectra. The shape of the permeate derivatives indicated the presence of low amounts of tryptophane, which was not detected when using a mass spectrometry method. The values of parameters characterising the derivatives of UV spectra of low-molecular weight fraction of β-casein hydrolysate differ significantly (from P < 0.05 to P < 0.001) from the analogous parameters determined in respect to the derivatives of spectra of other fractions of hydrolysate and from the derivatives of spectra of intact (non-hydrolyzed) protein. Using the UV spectroscopy method, it is possible to identify the low-molecular weight fraction of β-casein hydrolysate.