Abstract: Bovine milk β-lactoglobulin was found to induce proliferation of mouse resting spleen cells. Hence, the mitogenic activity was investigated in detail. The cell growth activity was measured by a colorimetric assay using 3-(4, 5-dimethylthiazol-2-yl)-2-5 diphenyl tetrazolium bromide (MTT). A reduced S-carboxymethylated β-lactoglobulin retained mitogenic activity towards mouse spleen cells and the mitogenic activity increased noticeably after digestion with gastrointestinal proteinases, in particular pancreatin. The pancreat n digest also induced proliferation of U266 cells, a human B cell line while it little influenced the growth of a human B cell line, Ball and human T cell lines Jurkat or Molt-3. These results suggest that bovine β-lactoglobulin may involve a mitogenic sequence(s) against some lymphocytes by the action of intestinal proteinases.