Vitamin B2 was intramuscularly administered to Holstein cattle and the ensuing changes in peripheral blood neutrophil function were investigated. The neutrophil count displayed a significant increase at 1-2 days after administration, while nitroblue tetrazolium reducing activity and phagocytic bactericidal activity were enhanced at 1-4 days after administration in calves and at 1-6 days after administration in adult cows. The increases in the neutrophil count and the activation of neutrophil functions were observed, being manifested at dosages of 10 mg/kg or greater for calves and 5 mg/kg or greater for adult cows.