The activator of the D-serine deaminase operon, the product of the dsdC gene, has been partially purified. It is reasonably stable to routine purification procedures in the presence of its ligand D-serine, but not in its absence. It loses activity upon dialysis in amino acid-free buffer, but activity is completely restored upon readdition of D-serine. It apparently functions purely as an activator, no repressor function could be demonstrated at suboptimal D-serine concentration. It is a transcriptional control element. The time required for in vitro transcription of D-serine deaminase mRNA, nearly 4 min, is similar to that for beta-galactosidase. Since the beta-galactosidase monomer is a much protein, this is surprisingly long.