We previously reported the in vitro selection of a general-purpose RNA-cleaving DNA enzyme that exhibits a catalytic efficiency (kcat/KM) exceeding that of any other known nucleic acid enzyme [Santoro, S. W. and Joyce, G. F. (1997) Proc. Natl. Acad. Sci. U.S.A. 94, 4262-4266]. This enzyme contains approximately 30 deoxynucleotides and can cleave almost any RNA substrate under simulated physiological conditions, recognizing the substrate through two Watson-Crick binding domains. The kinetics of cleavage under conditions of varying pH, choice of divalent metal cofactor, and divalent metal concentration are consistent with a chemical mechanism involving metal-assisted deprotonation of a 2'-hydroxyl of the substrate, leading to substrate cleavage. Kinetic measurements reveal that the enzyme strongly prefers cleavage of the substrate over ligation of the two cleavage products and that the enzyme's catalytic efficiency is limited by the rate of substrate binding. The enzyme displays a high level of substrate specificity, discriminating against RNAs that contain a single base mismatch within either of the two substrate-recognition domains. With appropriate design of the substrate-recognition domains, the enzyme exhibits a potent combination of high substrate sequence specificity and selectivity, high catalytic efficiency, and rapid catalytic turnover.