Coated micrometer-sized paramagnetic particles (PMPs) are readily available and widely used in immunoassays, mainly for separation and as a solid phase. We have described in a separate paper a model sandwich assay in which ≈1×10⁵ to 1×10⁶ PMPs (2.8 μm diameter) are immobilised on a plastic strip at the end of the assay. In this paper, we describe the design of an instrument that is capable of determining the number of PMPs on the plastic strip. The paper also describes a method of making standard plastic strips with known numbers of PMPs on them. A strip, when placed in a coil of wire in parallel with a capacitor, causes the resonant frequency of the coil to decrease because of the presence of the PMPs. The decrease in frequency relates directly to the number of PMPs on the strip. A circuit based on a voltage-controlled oscillator and a phase-locked loop is used to accurately measure the resonant frequency of the coil. The instrument is capable of detecting at least 1×10⁵ PMPs immobilised on a plastic strip and has a linear response (r=0.99) for up to at least 3.33×10⁶ PMPs. In terms of the iron content of the PMPs, the detection limit is ≈1.2 μg Fe in the paramagnetic particles and the sensitivity is ≈3 Hz per μg of Fe. The instrument is small and compact and together with a suitable magneto-immunoassay will have many applications, including near-patient monitoring.