Kefir is an acidic-alcoholic fermented milk product originating from the Caucasian mountains. Kefir has long been known for its probiotic health benefits, including its immunomodulatory effects. The objectives of this study were to investigate the properties of a fermented whey product and to examine the effects of kefir grains on the in vitro immune-modulation of human mast cell-1 (HMC-1). The results showed that the whey fermented by kefir grains contained the maximum lactic acid bacteria and yeast for 16 hr by 1.83×10 8 and 6.5×10 5 CFU/ml, respectively, and lactose and whey proteins were partially hydrolyzed. The experimental whey fermented by kefir grains exhibited an in vitro anti-inflammatory effect on the HMC-1 line for 8, 16, and 24 hr, and this effect induced the expression of interleukin (IL)-4 as a pro-inflammatory cytokine, but not for 48 hr by RT-PCR in HMC-1 cells. In addition, the same phenomenon was observed for the expression of IL-8 as a pro-inflammatory cytokine by the kefir-fermented whey during the same periods of 8-48 hr under the same conditions. These cytokines resulted in the production of IL-4 at 20-25 ng in HMC-1 cells for 8, 16, and 24 hr, whereas 5 ng was produced for 48 hr by the fermented whey. In contrast, IL-8 was produced at 15-20 ng in HMC-1 cells during 4, 8, 16, and 24 hr, while 7 ng was produced at 48 hr. It was concluded that the whey fermented by kefir grains possesses a potential anti-inflammatory function, which could be used for an industrial application as an ingredient of functional foods and pharmaceutical products.