Abstract: A dry radioimmunoassay test composition comprising a mixture of a radiolabelled form of a ligand, i.e., a hapten or an antigen, an antibody for the ligand, and a heavy metal cation, e.g., cupric ion, to inhibit the complex-forming reaction between the radiolabelled ligand and the antibody. A method for preparing the dry test composition is also provided. In use in a radioimmunoassay, the dry test composition is reconstituted with an aqueous liquid and combined with a test sample and a chelating agent to bind the heavy metal cation, permitting reaction among any ligand present in the sample, the radiolabelled ligand, and the antibody. A test kit for performing such radioimmunoassay is also provided.