A randomized double-blind placebo-controlled immunity study involving 99 healthy volunteers was performed to investigate the effect of poly-γ-glutamate (γ-PGA) on human natural killer (NK) cell activity in peripheral blood. The volunteers were randomly assigned to one of three groups and orally treated with solutions (25 mL) containing 0 mg (placebo), 250 mg (low dosage), or 500 mg (high dosage) of γ-PGA. Each volunteer took one dose every 12 hours for 8 weeks. Blood samples were drawn before the initial treatment and at the 4th and the 8th weeks of treatment. NK cell activity was assessed by measuring its degranulation, cytokine production, and cytotoxicity against the K562 cell line. Our results revealed that the cytotoxic activities of NK cells from the high-dosage γ-PGA group were significantly higher (P<0.05 for all comparisons) compared to the low dosage and placebo groups at weeks 4 and 8 after the initial treatment. This increase in the NK cell activity among peripheral blood mononuclear cells (PBMCs) of healthy individuals was also confirmed in vitro (as assessed by the degranulation and cytokine production). These results suggest that the oral administration of γ-PGA induces a cell-mediated immunity by increasing the NK cell activity in humans.