In this work, we develop a platform for assaying biomolecules involving the measurement of alternating current ac magnetoreduction of magnetic reagent mixed with a detected sample. The magnetic reagent contains magnetic nanoparticles coated with a given kind of antibody, which associates with conjugated biomolecules. Then, the biomolecules can be quantitatively detected by measuring the ac magnetoreduction of magnetic reagent. To achieve hyper-high-sensitivity assay, a system utilizing a high-transition-temperature rf superconducting quantum interference device (SQUID) as a sensor to probe the magnetoreduction of reagent due to the association between biomolecules and magnetic nanoparticles. Examples to assay human CRP are given to demonstrate the validity of the assay platform, as well as the hyper-high sensitivity.