Natural killer (NK) cells play a key role in host defense against microbial pathogens. Establishing the reference intervals (RIs) of NK cell functions would be valuable in assessing the immune status of hosts. We evaluated the NK cell activity in healthy adults. We further established and validated the RIs of representative NK cell functions. Flow cytometry was used to evaluate the cytokine production and CD107a degranulation of NM cells. Levels of soluble IFN-gamma in the culture supernatants were evaluated by ELISA. Our results demonstrated that the intracellular IFN-gamma production of NK cells was positively correlated with CD107a expression and soluble IFN-gamma levels. There were no significant differences in NK cell functions between different age and gender groups. The mean values and RIs of representative NK cell functions are as following: IFN-gamma(+) NK cells (%): 28.09 (11.3-51.95); CD107a(+) NK cells (%): 17.90 (9.852-27.56); soluble IFN-gamma (pg/ml): 330.4 (41.38-717.8). In addition, the intracellular IFN-gamma production and degranulation activity of NK cells in patients with colorectal cancer were significantly lower than that in healthy adults. Our study has established the RIs of NK cell functions in healthy adults, which might be used for monitoring the immune status of the hosts. (C) 2016 American Society for Histocompatibility and Immunogenetics. Published by Elsevier Inc. All rights reserved.