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We have developed a liquid phase immunoassay technique using the Brownian relaxation of magnetic markers. In order to achieve this, we first characterized the magnetic markers, which are usually agglomerates of magnetic nanoparticles. The magnetic properties of the agglomerates were analyzed, and parameters such as the Brownian and Neel relaxation times and the magnetic moment were obtained. Using these key parameters, we could determine the markers that were suitable for the liquid-phase immunoassay. Then, we performed the detection of a biological target called biotin. In this experiment, biological targets were fixed on the surface of large polymer beads that were a few μm in size. The bound and unbound (free) markers were magnetically distinguished without requiring a washing process to separate them. We used three magnetic sensors for the signal detection: a SQUID, an MR sensor, and a flux gate. The sensitivity of SQUID, MR sensor, and flux gate to the molecular-number concentration was estimated to be as high as 5.6×10-18 mol/ml, 2.8×10-16 mol/ml, and 2.8×10-16 mol/ml , respectively. We also showed that the sensitivity could be improved by using magnetic markers with a large magnetic moment.