Novel magnetic assay methods and systems. According to a preferred embodiment, a chromatographic medium, which preferably comprises a test strip, is provided that is designed to be contacted with a test solution having activated magnetic particles such that the solution flows bilaterally thereacross. A magnetic field, generated by a magnet or electromagnet, is selectively applied to the medium which causes the charged particles to become substantially bound at a site on the medium specified by the position of the magnet, to thus form a captured line or zone. In one preferred embodiment, the magnetic field is applied at the site on the medium at which the test solution is contacted. The degree of magnetic force applied to the membrane may be selectively adjusted to vary the width or surface area of the capture line or zone.