Abstract: A method of assaying for an analyte including the steps of: (i) passing a sample suspected of containing an analyte and reagents comprising a target ligand-analyte receptor conjugate and a detectable tracer containing a label for the analyte through filter apparatus containing a plurality of discrete flow zones wherein at least one zone functions as a capture zone having bonded thereto a receptor ligand for said target ligand; (ii) allowing the sample and accompanying reagents to incubate prior to passage through said at least one zone to facilitate formation of complex(es) of said conjugate and said at least one analyte in a liquid or fluid phase; and (iii) detecting the presence of analyte in the sample by activation of the label in said at least one zone after binding of the complex(es) conjugate to the associated receptor ligand(s).