The effects of freezing treatments on the activity of the lactobacilli-aggregating peptide QRCVNLQA were studied. The aggregation activity of the peptide solution was enhanced by rapid deep freezing in liquid nitrogen followed by storage for at least 1 d below -10 degrees C. The treated peptide strongly induced the aggregation of suspended cells of Lactobacillus rhamnosus GG to provide a clear supernatant after 1.5 h. The peptide was activated by oxidation of thiol groups, probably through dimerization of the peptide molecules to form disulfide bonds. Addition of the freeze-oxidized peptide to a suspension of lactobacilli promoted the adhesion of the lactobacilli to a monolayer of Caco-2 intestinal epithelial cells.