To utilize ultraviolet (UV) irradiation as a means to prepare hypo-allergenic food, we investigated the molecular weight profile, secondary structure content, fluorescence spectrum, and immunoglobulin (Ig) production-regulating activity of $eta$-lactoglobulin after UV-irradiation. UV-irradiation caused a change on the molecular size distribution, disruption of the ordered structure, and decrease of emission intensity of $eta$-lactoglobulin. The alteration on Igs production regulating activity of $eta$-lactoglobulin by UV-irradiation was observed in the mouse spleen lymphocytes system. These results suggest that UV-irradiation is effective for alteration of molecular properties and antigenicities of $eta$-lactoglobulin and such treatment should be useful for the preparation of low allergenic foods.