The major metabolic route for the synthesis of phosphoenolpyruvate is from 2-phosphoglycerate catalyzed by the enzyme enolase (EC 4.2.1.11). Enolase occurs at the converging point between glycolysis and gluconeogenesis and may be an important regulatory enzyme. Growth ofEscherichia coli JA 200 pLC 11-8 to stationary phase in low-phosphate medium containing32P-orthophosphate and glucose as the carbon source resulted in incorporation of label into the enzyme. In vivo labeling of enolase was demonstrated by immunoaffinity chromatography of the labeled crude extract. In addition,32P-enolase was identified with sodium dodecylsulfate polyacrylamide gels, two-dimensional gel electrophoresis, and Western blot analysis, followed by autoradiography.