Abstract l-Glutamate decarboxylase was purified from a sucrose-negative strain of Escherichia coli. The specific activity of 60% pure enzyme was 19,550 μl/10 minutes/mg. Borohydride reduction at pH 4.8–5.2 resulted in (a) the disappearance of the peak at 415 mμ, and appearance of an inflection at 330 mμ; (b) loss of 97% of the activity; (c) formation of a pyridoxyllysyl residue; and (d) disappearance of pyridoxal phosphate. Borohydride reduction at pH 7.2–7.6 caused loss of 50% of the activity. Formation of the pyridoxyllysyl residue was not observed. Pyridoxal phosphate was split off the enzyme after reduction at the higher pH. It is concluded that pyridoxal phosphate is bound as a Schiff base to a lysyl residue of the enzyme at low pH and is bound in a form at higher pH which is resistant to borohydride reduction.