<jats:p> <jats:sc>Duerre, John</jats:sc> A. (University of North Dakota, Grand Forks), <jats:sc>and Chris H. Miller</jats:sc> . Cleavage of <jats:italic>S</jats:italic> -ribosyl- <jats:sc>l</jats:sc> -homocysteine by extracts from <jats:italic>Escherichia coli</jats:italic> . J. Bacteriol. <jats:bold>91:</jats:bold> 1210–1217. 1966.—Cell-free extracts prepared from <jats:italic>Escherichia coli</jats:italic> catalyze the cleavage of the thioether linkage of <jats:italic>S</jats:italic> -ribosylhomocysteine. One of the products has been identified as homocysteine by paper chromatography of the <jats:italic>N</jats:italic> -ethyl-maleimide derivative and can be measured directly in reaction mixtures by sulfhydryl reagents. The other compound has been tentatively identified as ribose by column chromatography. Free sulfhydryl groups were also detected with use of <jats:italic>S</jats:italic> -adenosylhomocysteine as substrate; however, enzymatic cleavage of the glycosidic linkage of this compound appears to be required prior to cleavage of the thioether linkage. Homocysteine formed from either substrate could be converted readily to methionine, provided the necessary cofactors were added. Some of the properties of the ribosylhomocysteinase are discussed.