Cultured leukemic T-lymphoblasts, incubated in the presence of inhibitors of adenosine deaminase, are exquisitely sensitive to growth inhibition by deoxyadenosine. An analogy between this phenomenon and human combined immunodeficiency disease associated with inborn adenosine deaminase deficiency and the use of inhibitors of adenosine deaminase in the management of T-cell acute lymphoblastic leukemia has been noted. These phenomena are believed to reflect accumulation of high intracellular concentrations of deoxyadenosine triphosphate (dATP) following phosphorylation of deoxyadenosine, inhibiting replicating T-cells. In an attempt to extend these observations to noncultured, nonleukemic T-cells, we studied deoxyadenosine metabolism in human thymocytes. Human thymuses were separated into large replicating and small nondividing cell types by centrifugal elutriation. Both thymocyte subpopulations elevated in their dATP pools on incubation with microM concentrations of deoxyadenosine in the presence of erythro-9-[3-(2-hydroxynonyl)]adenosine, an inhibitor of adenosine deaminase. These dATP pool rises were similar in extent to those found in cultured leukemic T-lymphoblasts. However, the finding that small nonreplicating thymocytes elevate their dATP pool was unexpected. This prompted study of unstimulated peripheral blood lymphocytes. These cells (T and non-T) showed a similar elevation of their dATP pool on incubation with deoxyadenosine. Furthermore, these nondividing peripheral blood lymphocytes were killed by microM concentrations of deoxyadenosine in the presence of an inhibitor of adenosine deaminase. The biochemical mechanism of this G0-phase cell death is not known. These findings provide impetus for the investigation of adenosine deaminase inhibitors as lympholytic immunosuppressants or as agents to noncycling malignant lymphoid cells.