InMicrococcus lysodeikticus, the first enzyme of serine biosynthesis, phosphoglyceric acid dehydrogenase, was found to be inhibited byL-serine, adenosine-5′-monophosphate (AMP), the reduced form of nicotinamide adenine dinucleotide, and less strongly by glycine. Inhibition by serine was increased by the incorporation of glycerol or divalent cations into cell-free extracts, and was decreased by ammonium sulfate fractionation. Inhibition by AMP was not modified by glycerol or ammonium sulfate fractionation but was increased by added divalent cations. The addition of Ca2+ions resulted in a greater increase in serine inhibition than Mg2+, Mn2+, or Zn2+ions. Serine and AMP inhibitions approached maxima in extracts containing about 30 mMCa2+. The synthesis of labeledL-phosphoserine from14C-3-phosphoglyceric acid was also inhibited by serine and AMP.