An object of the present invention is to provide a method of reducing interference to a measurement system from water-soluble silicone and/or surfactant contaminating the measurement system in a latex agglutination immunoassay. The execution of a latex immunoagglutination reaction in the presence of a silicone compound can reduce the interference to the measurement system from a component derived from a micro blood-collection tube (water-soluble silicone) and/or surfactant mixed from outside of the measurement system.