The objective of the research performed was to determine the effect of Lactobacillus casei ŁOCK 900 probiotic strain on the proteolysis of protein in raw-ripening pork loins. The research material consisted of raw-ripening pork loins with the use of probiotic strain of Lactobacillus casei ŁOCK 900. Changes were assessed immediately after the completed aging and 2 months after cool storage. Three loin variants were prepared for investigation: a control sample without the probiotic, a sample with the probiotic, and a sample with the probiotic and sodium ascorbate added. There were determined: proteolysis index, water content of water-soluble nitrogen, free amino acid, water-soluble peptides; the antioxidant activity of protein proteolysis products towards ABTS^+· cation radical was assessed. The presence of probiotic strain during the aging process of raw-ripening pork loins caused the rate of protolytic changes during cool storage to be suppressed. The highest proteolysis index was found 2 months after cool storage in the control sample (10.96 %) and it was higher by 0.51 % compared to the sample with the probiotic and sodium ascorbate (P2) and by 0.52 % compared to the sample with the probiotic (P1). In the samples containing the probiotic added, 2 months after cool storage, the determined amount of free amino acids was lower (28.61 – 28.71 mg/g) compared to the control sample (29.02 mg/g). The control sample contained more peptides after aging (2.85 mg/g) and after storage (3.78 mg/g) versus the samples with the probiotic added (2.35 – 2.53 mg/g after aging and 3.24 – 3.44 mg/g after storage, respectively). After aging, the aqueous extracts of peptides in the samples with the probiotic were characterized by the highest antiradical activity of (1.4 – 1.7 mg Trolox/mg of peptides), and the free amino acids in the control sample were characterized by the lowest antiradical activity (0.03 mg Trolox/mg of amino acids).