Abstract: A carrier which consists of a carboxyl group-containing polymer, preferably activated polymethyl methacrylate, and on the surface of which a biologically active substance consisting of Poly I:C is immobilized, is described, as is a method of preparing such a carrier, and its use for diagnostic purposes. In the method of immobilizing Poly I:C on a carrier surface of polymethyl methacrylate, the surface of the polymethyl methacrylate carrier is first activated by treating it with strong alkali, such as a 10M sodium hydroxide solution. An aqueous solution of Poly I:C is heated, preferably for 1-2 h, and is then allowed to cool to about 50.degree.-80.degree. C., whereupon a coupler, preferably a water-soluble carbodiimide, is added to the solution. The warm solution is then applied to the activated carrier surface to immobilize Poly I:C on the carrier surface which then is allowed to cool to 0.degree.-50.degree. C.