Adenosine deaminase plays a key role in adenosine metabolism. This nueleoside has some important pharmacological and toxic effects. An adenosine deaminase deficiency observed in some cases of severe congenital immunodeficiency represents the first link between an immunological and enzymic defect. Adenosine deaminase from calf intestinal mucosa, the most widely studied enzyme, hydrolytically deaminates, in addition to adenosine, some other natural and synthetic nucleosides and also dehalogenates 6-halopurine ribosides. Many studies have been carried out to define the contribution of purine and ribose moieties in the binding of substrate and to help prepare effective inhibitors of the enzyme. This chapter examines the synthesis of 9-(p-Bromoacetamidobenzyl) adenine, inactivation of enzyme and stoichiometry of reaction, identification of alkylated amino acid residues, and kinetics of the reaction.