Glycyrrhizin (18$eta$-glycyrrhetinic acid-3-O-$eta$-D-glucuronopyranosyl-(1$ ightarrow2)-eta$-D-glucuronide, GL) was transformed to 18$eta$-glycyrrhetinic acid-3-O-$eta$-D-glucuronide (GAMG) by Streptococcus LJ-22. The antiallergic activities of GL and GAMG was measured using a RBL cell assay system and contact hypersensitivity model mice. GAMG exhibited anti-allergic activity with $IC_{50}$ values of 0.28 mM. GAMG, which is sweeter than GL, and 18$eta$-glycyrrhetinic acid, which is a GAMG metabolite by human intestinal bacteria, also inhibited the passive cutaneous anaphylaxis and skin contact inflammation. In conclusion, GAMG may be useful as a new sweet food additive and an anti-allergic agent.