Low activity of 5'-nucleotidase (5'-ribonucleotide phosphohydrolase, EC 3.1.3.5) in T lymphoblasts may explain the marked sensitivity of this cell to deoxynucleotide accumulation when compared to B Iymphoblasts. The relevance of such observations with cultured cells to the normal immune system requires the demonstration of similar differences in the 5'-nucleotidase activity of normal human lymphocyte subpopulations. Sheep erythrocyte (E) rosette-forming cells from normal thymus, tonsi , and peripheral mononuclear cells have 5'-nucleotidase activities of 1.7, 11.3, and 21.2 nmol/hr per 106 cells. Non-E-rosette forming cells from the peripheral blood or tonsil have 5'-nucleotidase activity comparable to the higher levels found in the peripheral E-RFC. Increased levels of5'-nucleotidase activity may be a marker for post-thymic T Iymphocytes.T lymphoblasts have 5'-nucleotidase activity similar to values demonstrated for E-RFC in thymus, whereas cultured B lymphoblasts have 5'-nucleotidase activity 15 times greater than that of T lymphoblasts. On the basis of these observations, the 5'-nucleotidase deficiency in congenital agammaglobulinemia has been reevaluated. In these patients the data indicate that peripheral E-rosette forming cells have the enzyme deficiency, demonstrating an abnormality of T lymphocytes in this disorder of immunoglobulin production. Lymphocyte 5'-nucleotidase (5'-ribonucleotide phosphohydrolase, EC 3.1.3.5) may have an important role in the regulation of the human immune system. Decreased activity of this purine catabolic enzyme has been observed in patients with primary immunoglobulin deficiency (1-4). However, it remains unclear whether an etiological relationship exists between the immune dysfunction and this enzyme deficiency.A marked difference in 5'-nucleotidase activity has been proposed as the molecular basis for the differential susceptibility of B and T lymphoblast lines to the toxicity related to deoxynucleoside accumulation (5-7). T lymphoblasts are killed by deoxyadenosine or deoxyguanosine, rapidly accumulate dATP or dGTP, and have very low levels of 5'-nucleotidase (5-9). In contrast, B lymphoblasts grow normally in deoxyadenosine or deoxyguanosine, accumulate only small quantities of deoxynucleoside triphosphates, and have higher 5'-nucleotidase levels than T lymphoblasts (5-9 and four T lymphoblast lines (CEM, Jurkat, MSB-2, and MOLT-3) were maintained and characterized as described (10).Peripheral lymphocytes were obtained, separated into Erosette forming cells and non-E-rosette forming cells, and characterized by described methods (11)(12)(13). Lymphoid organs were obtained from children with no known immune dysfunction, and cells were separated from these tissues within 1 hr of surgery by using described methods (14)