Activity hydrolyzing both N omega-phosphoarginine and glucose-6-phosphate was detected in rat renal microsome but not in hepatic microsome. Renal microsome was solubilized with 1% n-octyl-beta-D-thioglucoside and purified with DEAE-Sepharose column chromatography. Fractions hydrolyzing both N omega-phosphoarginine or glucose-6-phosphate were subjected to 7.5%-polyacrylamide gel electrophoresis in the presence of 0.1% sodium dodecyl sulfate. Phosphatase activity in the gels was detected by a lead nitrate stain using N omega-phosphoarginine or glucose-6-phosphate as substrates. Both substrates produced a stain in the region of the gel corresponding to a protein with a mass of 150 kDa. Extracts of slices from this region of the gel also hydrolyzed phosphocreatine, inorganic pyrophosphate, and O-phosphotyrosine. Moreover, the phosphatase had its optimal pH in the alkaline range and was inhibited completely by 20 microM sodium vanadate, 1 mM cysteine, and 1 mM tetramisole. All these properties indicate that the microsomal phosphoamidase (EC 3.9.1.1) of rat kidney was identical with alkaline phosphatase (EC 3.1.3.1).