An in vitro evolution procedure was used to obtain RNA enzymes with novel catalytic function. The procedure was initiated by generating a population of 10¹³ variants of the Tetrahymena ribozyme. This is a group I ribozyme that catalyzes sequence-specific cleavage of RNA via a phosphoester transfer mechanism. It also has a limited ability to cleave DNA, provided one employs conditions of high temperature or high MgCl₂ concentration, or both. A selection constraint was imposed on the population of ribozyme variants such that only those individuals that carried out DNA cleavage under physiologic conditions were amplified to produce "progeny" ribozymes. Mutations were introduced during amplification to maintain heterogeneity in the population. This process was repeated for ten successive generations, resulting in enhanced (100 times) DNA cleavage activity.