Abstract: Reagent and method for detecting the presence of hepatitis associated antigen (HAA) in blood serum. The reagent comprises anti-HAA antibodies coupled via an intermediate silane to a plurality of siliceous particles having a surface area of at least about 1 m.sup.2 /g and, if the particles are porous, an average pore diameter of at least about 1000 A. The test method comprises the steps of incubating a serum sample with the reagent to complex any HAA in the samplewith the antibodies of the reagent, incubating the reaction product with a solution of radio labelled anti-HAA antibodies to complex the labelled antibodies with any HAA that may be complexed to the reagent, separating the incubation products from the solution of labelled antibodies, and counting the radioactivity of either the separated products or the remaining solution to determine whether HAA was in the sample.