Natural killer (NM) cells are lymphocytes of the innate immune system and have the ability to kill tumor cells and virus-infected cells without prior sensitization. Malignant tumors and viruses have developed, however, strategies to suppress NI(cells to escape from their responses. Thus, the evaluation of NM cell activity (NKA) could be invaluable to estimate the status and the outcome of cancers, viral infections, and immune-mediated diseases. Established methods that measure NKA, such as Cr-51 release assay and CD107a degranulation assay, may be used to determine NI(cell function, but they are complicated and time-consuming because they require isolation of peripheral blood mononuclear cells (PBMC) or NK cells. In some cases these assays require hazardous material such as radioactive isotopes. To overcome these difficulties, we developed a simple assay that uses whole blood instead of PBMC or isolated NM cells. This novel assay is suitable for high-throughput screening and the monitoring of diseases, because it employs serum of ex vivo stimulated whole blood to detect interferon (IFN)-gamma secreted from NM cells as an indicator of NKA. After the stimulation of NM cells, the determination of IFN gamma concentration in serum samples by enzyme-linked immunosorbent assay (ELISA) provided a swift, uncomplicated, and high-throughput assay of NKA ex vivo. The NKA results microsatellite stable (MSS) colorectal cancer patients was showed significantly lower NKA, 263.6 +/- 54.5 pg/mL compared with healthy subjects, 867.5 +/- 50.2 pg/mL (p value <0.0001). Therefore, the NKA could be utilized as a supportive diagnostic marker for microsatellite stable (MSS) colorectal cancer. (c) 2014 Elsevier Inc. All rights reserved.