Nigerooligosaccharides (NOS), a mixture of nigerose and nigerosylmaltooligosaccharides, is immunopotentiating oligosaccharides found in foodstuffs. Augmentation of natural killer (NK) activity by NOS was studied in vitro and in vivo in mice. In vitro treatment of hepatic mononuclear cells (MNC) from normal mice with 1 micro/ml NOS 17 h or just prior to the cytotoxicity assay enhanced their cytotoxicity against YAC-1 cells. NK activity of hepatic MNC was also enhanced in mice injected intraperitoneally with 0.4 mg NOS or in mice given 1% NOS solution orally as a drinking fluid. In association with the augmentation of NK activity, increase in the liver weight induced by intravenous inoculation of EL-4 tumor cells was significantly suppressed by intraperitoneal pretreatment with 0.4 mg NOS. Moreover, drinking 1% NOS significantly improved the survival curve of mice intravenously inoculated with EL-4 cells. Our results suggest that the immunopotentiating activity of NOS is exerted partly through the augmentation of NK activity.