During the past decades, organic and inorganic colloids have been widely used for biological or biotechnological applications. Over the last ten years, colloidal organic/inorganic composites, namely super-paramagnetic particles, have constituted a real improvement for some of these applications. Very recently, we have developed a new way of using such colloidal composites in the fundamental domain of molecular recognition and in the closely linked but more industrial field of immunological diagnosis.