The paper presents a rapid immunoassay system capable of quantifying analyte in complex biological and environmental media. Antibody-coated micrometer-sized paramagnetic particles are used as labels in an assay in which they bind quantitatively with an analyte and capture antibody molecules immobilized on a polyester disk to form a sandwich assay. The assay is performed in a simple reaction vessel, and reactions between labels, analyte, and antibodies are accelerated by positioning magnets alternately above and below the vessel. The bound paramagnetic particles are quantified using a simple flat, spiral, coil located just below the polyester disk. The electronic circuitry associated with the coil uses components that are inexpensive and readily available. The coil has been designed to respond only to particles bound on the disk and not to unbound particles still in the test solution. Unbound particles are pulled away from the disk by the magnet before readings are taken. The use of the reaction vessel with the cardiac markers CRP and CKMB is described. No sample preparation or washing step is used in the assays, and results can be obtained in less than 3 min after introducing the sample into the vessel with sensitivities in the normal clinical range.