Proteose peptone (PP) is a heat-stable and acid-soluble protein in milk whey. We reveal in this study the IgE production-suppressing activity of the PP fraction in bovine milk. The PP fraction suppressed IgE production by human myeloma cell line U266 cells by depressing the IgE mRNA expression. The suppressive activity of the PP fraction was facilitated by trypsin digestion. An oral administration of the PP fraction significantly decreased the levels of total and ovalbumin (OVA)-specific IgE in the serum collected from OVA-sensitized mice. However, the serum levels of other Ig classes in OVA-sensitized mice were not affected by the intake of the PP fraction. The PP fraction suppressed the mRNA expression level of IgE in mice splenocytes collected from OVA-sensitized mice. Moreover, the B cell population in the spleen was decreased, while the T cell population was increased by administering the PP fraction. These results suggest that the PP fraction modified the B/T cell balance.