This study was performed to determine the effects of $gamma$-irradiation on the antiallergic activity of alginate. An alginate aqueous solution was $gamma$-irradiated at 3, 5, 7, 10, 20, and 100 kGy. First, the molecular weight (Mw) of alginate rapidly decreased as the $gamma$-irradiation dose increased up to 20 kGy. Then, the antiallergic activity of the $gamma$-irradiated alginate was measured. Interlukin (IL)-4 cytokine and IgE level were significantly decreased in the $gamma$-irradiated alginate groups as compared to the control group in vitro. The mice intraperitoneally administered with the $gamma$-irradiated alginate exhibited lower ovalbumin-specific IgE and IgG1 level in serum than the control mice. Furthermore, the $gamma$-irradiated alginate suppressed total and ovalbumin-specific IgE secretions in the splenocytes. Increased IL-2 level was observed in the culture supernatants of the splenocytes that were obtained from the mice administerd with the $gamma$-irradiated alginate, while IL-4 level decreased. The present study indicates that $gamma$-irradiated alginate can suppress allergy in a mouse allergy model.