A fast and versatile technique has been developed for detecting small quantities of specific microorganisms or molecules with high specificity. The target analytes are bound to a substrate and placed in the measurement cell of a microscope based on a high-transition temperature Superconducting Quantum Interference Device (SQUID). A solution containing nanometer-size magnetite particles, coated with antibodies specific to the target, is added. The particles, which bind to the target via the antibody- antigen interaction, are superparamagnetic with a Neel relaxation time of ~1s. A pulsed magnetic field aligns the dipole moments, and the SQUID measures the magnetic relaxation signal when the field is turned off. Unbound magnetic particles relax rapidly (~15microsecond(s) ) by Brownian rotation and are not detected. On the other hand, particles bound to targets cannot rotate and instead relax slowly by the Neel mechanism. As a result, only bound particles contribute to the signal, allowing for quantification of the number of targets present without the need for a wash step. The current system can detect as few as 2000 magnetic particles. This technique could be used to detect a wide range of bacteria, viruses, and molecules, with potential applications in the food industry, clinical settings, or research laboratories.