Abstract: A solid-phase immunoassay support is provided by treating a test tube liner such as a preformed soft gelatin capsule half with a coupling solution comprising a fixative, a bifunctional coupling agent and a bioactive protein which selectively reacts with the substance which is the subject of the immunoassay. The support is particularly applicable to the radioimmunoassay (RIA) technique for determining the presence and concentration of minute amounts of protein antigens in biological fluid samples and can be readily inserted in an appropriately sized test-tube for easy and quick assays.