The HPRT gene mutation assay is a remarkable tool for testing genotoxic chemicals, allows for the isolation and screening mutation in different living cells. Here, we report the applicability of HGPRT gene to test the mutagenic activity of different concentrations of monosodium glutamate, MSG, (1, 3, 5 and 7 g/l). The mutagenic effect of monosodium glutamate was assessed by decreasing the viability and increasing the HGPRT gene mutation rate of Coprinopsis cinerea. The alterations were proportional to the concentration of MSG up to reach the optimum concentration (7 g/l) when the maximum rate of mutation was 1.7 and the viability was about 27%. The highest viability was 37.4% when oidia treated with 2 g/l of MSG. To determine the optimal mutagenic time of the optimal mutagenic concentration, the period of incubation was ranging from 1 to 4 hours of the cells treated with 7 g/l MSG. Two hours was the optimal mutagenic time that reaches 1.02 mutagenic rate of HGPRT gene mutation and 37.7% of the viability. The highest viability was gained is 43.6% when cells treated with 7 g/l MSG and incubated for one hour. In vitro results indicates that the MSG is mutagenic and subsequently may cause DNA damage. These data do provide an indication of potential genotoxic of MSG to human health. Thus its use as a food additive should be completely avoided and look for a safer alternative.