To search for the most effective dietary n-3/n-6 ratio to suppress the type I allergic response, we performed basic experiments that applied parameters, associated with the type I allergy. Guinea pigs fed on diets containing lipids with the n-3/n-6 ratio at different levels and the polyunsaturated fatty acid/saturated fatty acid ratio of a fixed level were sensitized with ovalbumin and reared for two weeks. The lowest or critical level of the n-3/n-6 ratio which produced a significant difference in the parameters was as follows: about 2.0 for the response of mast cells and eosinophils; 0.5 and 1.0, respectively, for the uptake of n-3 and n-6 polyunsaturated fatty acids and decreased histamine production; and 0.2 for decreased leukotriene B4 and total leukotrienes 4, and increased leukotrienes 5/leukotrienes 4. The critical level of the n-3/n-6 ratio thus differed widely according to the parameter. Overall, the upper limit for the dietary n-3/n-6 ratio to suppress antigen-induced type I allergic responses is suggested to be around 1.0.