The levels of maternal immunity enhancing factor(MIEF), which is an immunomodulatory protein identified from bovine colostrum, were determined by indirect competitive enzyme-linked immunosorbent assay(ELISA) for the colostrum and normal milk collected during the first two weeks of lactation. The mean concentration of MIEF in the colostrum of the first day of lactation was $109;{mu}g/ml$, and fell from the third day of lactation to $3{sim}4;{mu}g/ml$. The molecular weight of the purified MIEF determined by reducing SDS-PAGE and TSK G2000SW column chromatography was 22,000 and 24,000 daltons, respectively, showing that MIEF is a monomeric peptide in its native form. To examine the capacity of MIEF to induce differentiation of B Lymphocytes, human tonsillar Iymphocytes were cultured in the presence of different concentrations of MIEF, and then antibody secreting cells were enumerated by enzyme-linked immunospot(ELISPOT) assay. When added to cultures of human tonsillar Lymphocytes, MIEF induced differentiation of resting B Iymphocyte to antibody secreting plasma cells as efficiently as LPS.