The in vitro and in vivo effects of riboflavin sodium phosphate (vitamin B2 sodium phosphate: VB2Na) on host defense mechanisms, especially macrophage function, were investigated in mice. VB2Na (100mg/kg) was injected intraperitoneally or intravenously at 1 day before the phagocytosis assay. After intraperitoneal administration, the phagocytic index of