Lymphocytes from three infants with reticuloendotheliosis and eosinophilia ( Omenn 's syndrome) and immunodeficiency were assayed for 5′-nucleotidase activity. B and T lymphocytes from all three patients were totally deficient in ecto-5′-nucleotidase activity, but had normal levels of cytoplasmic 5′-nucleotidase. In contrast, cultured lymphocytes expressed normal ectoplasmic and cytoplasmic activities, suggesting that the lymphocyte-restricted enzyme deficiency was not likely a primary genetic defect. The deficiency of lymphocyte ecto-5′- nucleotidase was not associated with any abnormality of deoxynucleoside metabolism. The absence of lymphocyte ecto-5′-nucleotidase may be a characteristic feature of this syndrome and may help to distinguish this disease from others with similar manifestations.