OM-X® is a hand-made and naturally manufactured probiotic supplement. This fermented food product is made from vegetables, fruits, seaweeds and mushrooms, using 12 strains of lactic acid bacteria and bifidobacteria. OM-X® is also known to have beneficial health properties, and some of its components show effects on antigen (Ag)-stimulated degranulation activity, indicating that OM-X® may be useful in the treatment of allergy responses and symptoms. In this study, we evaluated the inhibitory effects of OM-X® on Ag-stimulated degranulation in rat basophilic leukemia RBL-2H3 cells, clarified the underlying mechanisms, and determined the active compounds in OM-X® for suppression of degranulation. Treatment with OM-X® gradually suppressed Ag-stimulated degranulation throughout the maturation period. OM-X® also gradually produced melanoidins by lactic acid bacterial fermentation during the maturation process. There was a high correlation between the suppression levels of Ag-stimulated degranulation and the browning of OM-X®. Furthermore, the inhibition of Ag-stimulated degranulation by OM-X® was found to be partially due to the direct inactivation of NADPH oxidase. To elucidate the in vivo effects of OM- X®, type I allergy model mice were orally administered with OM-X®, and the passive cutaneous anaphylaxis (PCA) reaction was measured. OM-X® intake remarkably suppressed the PCA reaction. Taken together, our findings suggest that OMX® could be a beneficial food to ameliorate allergic reactions.