Abstract O-Acetylserine sulfhydrylase-A is one of two highly specific enzymic activities which catalyzes the final step in l-cysteine biosynthesis in Salmonella typhimurium: O-acetyl-l-serine + sulfide → l-cysteine + acetate. The enzyme has been purified to near homogeneity and characterized. It is a pyridoxal phosphate-containing protein of s20,w 4.5 and molecular weight 68,000. Equilibrium sedimentation studies on native and carboxymethylated enzyme, analytical polyacrylamide disc gel electrophoresis in urea, pyridoxal phosphate analysis, tryptic peptide mapping, and amino acid analysis have shown that O-acetylserine sulfhydrylase-A consists of two identical subunits of molecular weight 34,000. The spectrum of native O-acetylserine sulfhydrylase-A has a visible absorption maximum at 412 mµ due to pyridoxal phosphate. When O-acetyl-l-serine is added to the enzyme, the visible absorption maximum shifts to 470 mµ. This shift is dependent upon the concentration of O-acetyl-l-serine, and the equilibrium constant, determined spectroscopically, for this reaction is 6 x 10-7 m. The shift in spectral absorption does not occur in the presence of sulfide. The Km for O-acetyl-l-serine, measured kinetically, in the presence of both substrates is 5 x 10-3 m.