Inherited deficiencies of the enzymes adenosine deaminase (adenosine aminohydrolase; EC 3.5.4.4) and purine nucleoside phosphorylase (purine-nucleoside:orthophosphate ribosyltransferase; EC 2.4.2.1) preferentially interfere with lymphocyte development while sparing most other organ systems. Previous experiments have shown that through the action of specific kinases, nucleosides can be “trapped” intracellularly in the form of 5′-phosphates. We therefore measured the ability of newborn human tissues to phosphorylate adenosine and deoxyadenosine, the substrate of adenosine deaminase, and also inosine, deoxyinosine, guanosine, and deoxyguanosine, the substrates of purine nucleoside phosphorylase. Substantial activities of adenosine kinase were found in all tissues studied, while guanosine and inosine kinases were detected in none. However, the ability to phosphorylate deoxyadenosine, deoxyinosine, and deoxyguanosine was largely confined to lymphocytes. Adenosine deaminase, but not purine nucleoside phosphorylase, showed a similar lymphoid predominance. Other experiments showed that deoxyadenosine, deoxyinosine, and deoxyguanosine were toxic to human lymphoid cells. The toxicity of deoxyadenosine was reversed by the addition of deoxycytidine, but not uridine, to the culture medium. Based upon these and other experiments, we propose that in adenosine deaminase and purine nucleoside phosphorylase deficiency, toxic deoxyribonucleosides produced by many tissues are selectively trapped in lymphocytes by phosphorylating enzyme(s).