Summary Three forms of calf intestine adenosine deaminase were resolved by gel filtrat in Sephacryl C-200 into species of 38,000 (E S ), 57,000 (E I ) and greater than 200,00 (E L ) daltons. The sensitivities of the three forms of adenosine deaminase to the adenosine deaminase inhibitors, 2′-deoxycoformycin (dCF) and erythro-9-(2-hydroxynonyl)adenine (EHNA) were measured. The concentration of inhibitor producing 50% inhibition of E L , E I , and E S was 2 × 10 −8 M, 8 × 10 −11 M and 2 × 10 −11 M, respectively for dCF and 1 × 10 −7 M, 3 × 10 −10 M and 2 × 10 −11 M, respectively, for EHNA. Thus, the E S and E I forms of adenosine deaminase could be distinguished from the E L form the basis of their respective sensitivities to dCF and EHNA.