Resistant glucan mixture (RGM), a water-soluble dietary fiber produced by the random polymerization of glucose with activated carbon as a catalyst at a high temperature, has been recently developed by our group. There has been little physiological and safety research into RGM and therefore we now present our research into its safety. A reverse mutation assay indicated that RGM is not mutagenic either with or without metabolic activation. We conducted a 90-day subchronic oral toxicity study in rats. Male and female rats fed either a 3% or 5% w/w RGM diet had no muddy or watery stools, and there was no RGM-related death in any group. Although some parameters in the 3% and 5% w/w groups were significantly different from those in the control group, these changes were not due to any toxicity from RGM. The results indicated that the No Observed Adverse Effect Level (NOAEL) of RGM was 3.3 and 3.9 g/kg body weight (BW) per day in male and female rats, respectively. We then studied the gastrointestinal effects of RGM in healthy adult humans. Gastrointestinal symptoms, such as gurgling sounds, flatus and tenesmus, were mild and transient. In men and women, the maximum no-effect dose for diarrhea was more than 0.9 g RGM /kg BW. The results of our current safety assessment studies suggest that RGM is safe for human consumption.