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Magnetic immunoassays utilizing magnetic markers and a high-Tc SQUID have been performed. We first showed a design of the SQUID for the sensitive detection of the magnetic signal from the marker, where the spatial distribution of the signal field was taken into account. Using the design, we can obtain the relationship between the magnetization of the maker and the signal flux detected with the SQUID. This relationship is important for quantitative evaluation of the immunoassay. Next, we detected biological materials called IgE with a remanence measurement method, where a marker made of Fe3O4 particles with diameter of 25 nm was used. We showed that 2 atto-mol of IgE can be detected, which shows high sensitivity of the present method.