Rice fermented seasoning (RFS) enhanced IgM production by human peripheral blood lymphocytes (PBL) in vitro. The IgM production by the PBL cultured in serum-free medium supplemented with 3% RFS was enhanced about 3-fold. However, rice fermented extracts did not enhance IgG production by human PBL cultured under similar conditions. RFS showed no mitogenic activity toward the PBL in this study. These results indicate that RFS stimulates immunoglobulin productivity of each IgM-producing PBL.