Abstract: A novel material and device useful in solid-phase binding assays to determine the presence or amount of an analyte in a test sample, particularly antigens or antibodies, is disclosed. The material comprises a porous matrix of fibers and a plurality of substantially spherical, solid particles having an average diameter of from about 0.1 to about 5 microns. The particles are retained and immobilized upon the fibers of the matrix. Preferably, the particles have on their surfaces a substance capable of reaction with the analyte in the sample, and the average diameter of the particles is less than the average pore size of the matrix. The device, in a preferred embodiment, comprises a substantially planar layer of the described material.