A method for use in a flow matrix, which utilizes biospecific affinity reactions to detect an analyte in the sample, and which comprises allowing the sample comprising the analyte and an analytically detectable reactant (Reactant*) to migrate through flow channels in a flow matrix to a detection zone located in the matrix, in which there is a firmly anchored biospecific affinity reactant (Capturer), and capturing the Reactant* in the detection zone in an amount related to the amount of analyte in the sample. The Reactant* has labeled particles of an analytically detectable group, and the Capturer is anchored to the matrix by immobilized particles which exhibit hydrophilic groups on their surface. A test kit comprises a flow matrix having a detection zone in which there is a firmly anchored biospecific affinity reactant (Capturer), and an analytically detectable reactant (Reactant*). The Reactant* has labeled particles of an analytically detectable group, and the Capturer is anchored to the matrix by immobilized particles which exhibit hydrophilic groups on their surface.