A double-stranded RNA adenosine deaminase that catalyzes the conversion of adenosines to inosines in duplex RNA substrates was purified to near homogeneity from Xenopus laevis eggs. The final specific activity was approximately 2.0 nmol of inosine min-1 mg-1 at 25 degrees C and pH 7.9 with a 794-base pair RNA substrate. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis revealed a single major approximately 120-kDa protein band by silver staining. The purified enzyme migrated with an apparent molecular mass of 90 +/- 10 kDa during high performance liquid chromatography. Gel filtration of the partially purified enzyme gave an apparent molecular mass of 210 +/- 20 kDa, suggesting that the enzyme may dimerize or associate with other cellular components. Substrate modification was inhibited by excess substrate, thiol reagents, heparin, and moderate concentrations of monovalent cations.