The realization of biomolecular detection assays for diagnostic purposes is technologically very challenging because such tests demand full integration for ease of use and need to deliver a high analytical performance with cost-effective use of materials. In this article an optomagnetic immunoassay technology is described based on nanoparticles that are magnetically actuated and optically detected in a stationary sample fluid. The dynamic control of nanoparticles by magnetic fields impacts the key immunoassay process steps, giving unprecedented speed, assay control and seamless integration of the total test. The optical detection yields sensitive and multiplexed assays in a low-cost disposable cartridge. We demonstrate that the optomagnetic technology enables high-sensitivity one-step assays in blood serum/plasma and whole saliva. Drugs of abuse are detected at sub-nanogram per millilitre levels in a total assay time of 1 min, and the cardiac marker troponin I is detected at sub-picomole per litre concentrations in a few minutes. The optomagnetic technology is fundamentally suited for high-performance integrated testing and is expected to open a new paradigm in biosensing.