The use of heat treatment to purify enzymes by selective denaturation and then by the subsequent precipitation of denatured protein is a simple, rapid, and well established procedure. Successful applications are limited to those few enzymes that possess a thermostability considerably higher than the majority of cell proteins. The introduction of thermostable enzymes into the protein population of a mesophile by cloning offers a clear opportunity to employ a heat-treatment method of purification to its full advantage (e.g., see references 1 and 2). In light of the difficulties involved in purifying bacterial cellulases, the cloning of some of the cellulase and hemicellulase genes of Caldocellum saccharolyticum into Escherichia coli has provided a welcome alternative procedure for obtaining pure enzymes.