The alkaline phosphatase (EC associated with the outer membrane of was solubilized from a crude membrane preparation with detergent and purified 219-fold using ion-exchange chromatography and gel filtration. The yield of the purified enzyme was 21% and the specific activity was 438 units mg. The enzyme was most active at pH 8·5, readily hydrolysed 5'-, 2'- and 3'-ribose and deoxyribose nucleotides, glucose 6-phosphate, glycerophosphates and -nitrophenylphosphate and was strongly inhibited by EDTA and 8-hydroxyquinoline. The EDTA-inhibited enzyme could be reactivated to some extent by CoCl and more effectively by ZnCl. The phosphatase was slightly activated by -nitrophenylphosphate and from kinetic studies using this substrate two values, 0·56 x 10 and 3·4 x 10 , were estimated. The enzyme retained activity in the presence of SDS, unless it was heated, and had an apparent of 69 000, as determined by SDS-PAGE. Gel filtration data suggested that the native enzyme might consist of at least two subunits. The properties of the enzyme were consistent with the view that it is held in the outer membrane by hydrophobic interactions