ABSTRACTIncreased adenosine deaminase activity was observed in peripheral lymphocytes from a patient with adult T cell leukemia. Gel filtration pattern on a Sephadex G‐150 revealed that the enzyme from normal lymphocytes existed in two forms with different molecular weights (large and small), whereas the enzyme from the patient existed solely as the small molecular weight form. Comparison of this small form adenosine deaminase from the patient with that from normal controls revealed that both enzymes were quite similar in relative substrate specificity, Km values for adenosine, pH optima, heat stability pattern, and sensitivity to inhibition by coformycin, a tight binding inhibitor of adenosine deaminase. Increased adenosine deaminase activity and the lack of the large form of the enzyme might be significant features of adult T cell leukemia lymphocyte