Chemical modification studies of proteins originated when interest in quantitative determination of proteins and their various constituent amino acids was started. Later, chemical modification procedures were used to identify the particular amino acid residues required for the biological activity of proteins (1). The increasing interest in the subject during the last decade has been promoted by practical interests related, for example, to possible pharmacological or medical diagnostic application such as to convert a number of protein toxins into toxoids. These toxoids retain some of the original antigenic determinants but are no longer toxic. Other enzymes are modified chemically to alter and improve their native properties and endow them with useful new functions such as to make them more soluble and active and more stable in organic solvents and to change the selectivity of the enzyme (2). Modifications strategies now being developed should soon yield a wide spectrum of novel biomolecules whose activities are optimized for specific industrial processes or therapeutic applications.