An immunoassay device has a housing defining a first opening for receiving a solution and a second opening through which a liquid sample is deposited, a strip of sorbent material having a test site with immobilized antigen or antibody for the ligand to be tested, and a filter for filtering the sample. The filter is located at the second opening and directly above the test site. The sorbent material defines a horizontal flow path in the housing for the solution from the first opening to the test site. In use, the sample is first applied via the filter to the test site, and then, after the ligand has been captured, a buffer added through the first opening is used to cause a secondary specific binder conjugated to a marker to migrate horizontally by capillary action to the test site where it can bind to the already captured ligand. This immunoassay offer several advantages over the traditional chromatographic immunoassays.