Bovine colostrum and milk contain many immunomodulatory components. The low-molecular-weight fraction (<10 kda)="" was="" separated="" from="" colostrum="" and="" milk="" by="" gel="" filtration="" chromatography,="" and="" its="" effect="" on="" the="" oxidative="" burst="" of="" bovine="" polymorphonuclear="" leukocytes="" (pmnl)="" was="" investigated="">in vitro. The oxidative burst activity induced by Staphylococcus aureus was considerably enhanced when PMNLs were incubated with this low-molecular-weight fraction. However, phorbol 12-myristate 13-acetate did not trigger a burst after priming with this fraction. The oxidative burst activity enhanced by this fraction was reduced after heating. These results confirmed that a low-molecular-weight substance(s) of less than 10 kDa, present in bovine milk and colostrum, enhances the oxidative burst activity of PMNL.