In Escherichia coli, the enzyme called cysteine desulfhydrase (CD), which is responsible for L-cysteine degradation, was investigated by native-PAGE and CD activity staining of crude cell extracts. Analyses with gene-disrupted mutants showed that CD activity resulted from two enzymes: tryptophanase (TNase) encoded by tnaA and cystathionine P-lyase (CBL) encoded by metC. It was also found that TNase synthesis was induced by the presence Of L-Cysteine. The tnaA and metC mutants transformed with the plasmid containing the gene for feedback-insensitive serine acetyltransferase exhibited higher L-Cysteine productivity than the wild-type strain carrying the same plasmid. These results indicated that TNase and CBL did act on L-Cysteine degradation in E. coli cells.