Adenosine deaminase activity, with arabinosyladenine (ara-A) used as the substrate, was measured in human leukemic cells and major tissues of man, rat, and mouse. The enzyme is stable in frozen tissues for at least 1 week, in frozen homogenate for several months, or after overnight dialysis at 4 degrees C in 0.25 M sucrose. The adenosine deaminase of chronic myelogenous leukemic (CML) cells has a Km of 0.6 mM and a Vmax of 183 mumols x 10(-2)/g of cells/minute. All species have highest activities in spleen and intestine. Kidney activity is highest in rat and lowest in man. The human brain has higher activity than the mouse or rat brain, and activity is higher in mouse liver than in human or rat liver. Activity in lung tissues is moderate and similar in all three species. The mean activity in CML cells is 195 mumols x 10(-2)/g of cells/minute, and the activity is higher in patients with CML in blast cell crisis. Acute myelogenous and lymphocytic leukemia cells also have very high activities. Drug concentrations required for inhibiting 50% of CML enzyme activity are 10 nM or erythro-9-(2-hydroxy-3-nonyl) adenine (EHNA), 0.6 nM of coformycin, and 0.3 nM of 2'-deoxycoformycin. Preincubation of the enzyme with coformycin, but not with EHNA, enhances the inhibition. In view of the high adenosine deaminase activities in tissues and the enhancement of the therapeutic effect of ara-A by inhibitors in mouse tumor systems, clinical trials of these combinations are warranted. They may be particularly beneficial to patients with CML in blast cell crisis.