Abstract: There is disclosed an effective direct-acting iodinating reagent for biological specimens which, because it is direct, avoids the necessity of having present at the same time both an oxidizing agent and sodium iodide. The reagent is a conjugate of an iodous ion and a water insoluble support. The manner of conjugation is such that the ion is complexed onto the surface of the support but is still free to react with groups in proteins susceptible to iodination and thus be transferred to the biological specimen being iodinated. Apart from the advantage of being direct-acting, the reagent has those desirable attributes which are associated with water insoluble oxidants.