In this study, Adenosine Deaminase (ADA) activities were measured in sera from 29 patients with acute lymphocytic leukemia (ALL) and from 27 control subjects. No meaningful difference was observed between ADA activities of leukemic patients and control subjects. However, significant differences were found between serum enzyme activities of B-ALL and T-ALL patients. ADA activity was found higher in sera from T-cell ALL group compared with those of control and B-cell ALL groups. Furthermore, ADA activity was also found higher in serum from cALLA (+) B-cell ALL group than those of cALLA (-) B-cell ALL group.