Proof of principle has been shown for a rapid, quantitative, homogeneous immunoassay capable of analyzing whole-blood samples. The assay was performed with test cards and a small instrument designed for use at the point of care. The immunoassay has an immunological "front end" combined with a coagulation cascade chemistry "back end" and is made possible by combining two patented technologies: (a) a serine protease inhibitor [Porter and Bruhnke, Photochem and Photobiol 1990; 51(1):37] and (b) paramagnetic iron oxide particles (PIOP) in a mixture of buffers and coagulation assay components supplied as a dry film in a test-card reaction chamber [Oberhardt et al., Clin Chem 1991;37:520]. A model steric-hindrance immunoassay based on these technologies was established for the measurement of biotin. The calibration curve was developed by measuring plasma samples supplemented with biotin. The reagents were inhibited biotinylated thrombin, anti-biotin monoclonal antibody, and PIOP.