Adenosine deaminase has been measured by the indirect spectrophotometric method in lymphoblasts from adult and pediatric lymphoblastic leukemias, peripheral T- and B-cells, mature T-cell leukemia (Sézary cell), and mitogen-stimulated peripheral control T- and B-cells. It was found that, if the specific activity of the enzyme is expressed in terms of cell cytoplasmic protein concentration rather than cell number, null-cell leukemia has enzyme levels markedly higher than T-cell lymphoblastic leukemic or Sézary cells. The range in values is far less than those reported by other authors using the direct spectrophotometric assay and cell number prior to lysis. Non-leukemic T- and B-cells from control individuals show minor differences as do T gamma-enriched cells. Lymphocytes from female oriental control populations were significantly lower. Both mitogen-stimulated lymphoblasts and control lymphocytes show similar elevations in adenosine deaminase activity so that no effect per se can be ascribed to lymphoblastic transformation.