In many human tissues adenosine deaminase exists as a large molecular weight complex (large form) composed of adenosine deaminase and an adenosine deaminase binding protein. The molar ratio of adenosine deaminase to binding protein in this large form complex appears to be 2:1, respectively, based on several observations. Scatchard-type analysis of the binding of 125I-labeled adenosine deaminase to purified binding protein indicates that 2.15 mol of adenosine deaminase are bound to 1 mol of binding protein. Chemical cross-linking of 125I-labeled adenosine deaminase-binding protein complex (large form) with glutaraldehyde produces 6 cross-linked species with molecular weights consistent with the proposed 2 to 1 stoichiometry. Sedimentation equilibrium analyses reveal a native molecular weight of 300 890 for the adenosine deaminase-binding protein complex (large form), 37 500 for small form adenosine deaminase, and 213 300 for the binding protein. A 2:1 molar ratio of adenosine deaminase and binding protein in the large form complex is most consistent with these molecular weight estimates.