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Peptide composition, antioxidant activity, antimutagenic action and antimicrobial potential of defatted and fermented colostrum were compared. In colostrum samples fermented with Lactobacillus acidophilus the relative amount of fraction with a molecular mass (mr) under 10 kDa reached 7.5 %, which was directly correlated with the action of the bacterial proteolytic system. According to chromato-mass-spectrometry the fraction with mr 1200–1300 Da, or peptides consisting of 11–12 amino acid residues, predominated in fermented colostrum. The fluorimetric technique has revealed that antioxidant activity of fermented colostrum ultrafiltrate (fraction with mr ≤10 kDa) is double that of defatted sample. The Ames test demonstrated 9.5–15.3 % reduction of mutation rate caused by fermented colostrum in strain Salmonella typhimurium TA 98 and 4.6–11.1 % decrease in strain TA 100, exceeding the corresponding effects induced by defatted colostrum. It was found by impedimetric method that colostrum peptides showed a greater antibacterial potential against Gram-negative test strain Escherichia coli ATCC 8739 versus Gram-positive strain Staphylococcus aureus ATCC 6538. Fermentation of first milk with Lactobacillus acidophilus resulted in enhanced antioxidant, antimutagenic activities and production of peptides with antibacterial capacity. [ABSTRACT FROM AUTHOR]