The recently discovered association of adenosine deaminase (ADA) deficiency and combined immune deficiency (CID) has emphasized the critical role of purine salvage in the function of lymphoid tissue. Known enzymatic properties of ADA are presented. In addition, known phenotypic data and possible genetic mechanisms for the occurrence of ADA deficiency in CID are discussed. A hypothesis based on considerations of known metabolic pathways in human erythrocytes is proposed to account for the selectivity of ADA deficiency for lymphoid tissue. Finally, some inhibitors of ADA are discussed as well as some immunosuppressive agents.