The paper describes a method for trace iodination of immunoglobulins and other serum proteins using lactoperoxidase, hydrogen peroxide, and iodide. The method is gentle and rapid, avoiding protein denaturation, which is a common issue with other iodination methods. The study demonstrates that the method effectively iodinates tyrosyl residues in proteins, as evidenced by tryptic hydrolysis and peptide mapping. The susceptibility of proteins to iodination varies, with human γG immunoglobulin being more easily iodinated than α2-macroglobulin. Lactoperoxidase is found to be more efficient than horseradish peroxidase in iodinating proteins. The method is useful for preparing radioiodinated proteins for biochemical and immunological studies, providing high specific radioactivity without denaturation.The paper describes a method for trace iodination of immunoglobulins and other serum proteins using lactoperoxidase, hydrogen peroxide, and iodide. The method is gentle and rapid, avoiding protein denaturation, which is a common issue with other iodination methods. The study demonstrates that the method effectively iodinates tyrosyl residues in proteins, as evidenced by tryptic hydrolysis and peptide mapping. The susceptibility of proteins to iodination varies, with human γG immunoglobulin being more easily iodinated than α2-macroglobulin. Lactoperoxidase is found to be more efficient than horseradish peroxidase in iodinating proteins. The method is useful for preparing radioiodinated proteins for biochemical and immunological studies, providing high specific radioactivity without denaturation.