A selective nucleoside phosphorylating enzyme was purified to homogeneity from Morganella morganii NCIMB10466 crude extract. The enzyme appeared to consist of six subunits identical in molecular mass (M(r) 25,000). It phosphorylated various nucleosides at the 5'-position to produce nucleoside-5'-monophosphates using pyrophosphate as the phosphate source. Energy-rich compounds, such as carbamylphosphate and acetylphosphate, were also very effective phosphate donors. The enzyme also exhibited phosphatase activity, and dephosphorylated various phosphate esters, but had a weak effect on nucleoside-3'-monophosphates. Based on the results of the kinetic study, the enzyme appeared to be an acid phosphatase. Its activity was partly inhibited by sulfhydryl reagents and heavy metal ions, but not by chelating reagents such as EDTA. Using the purified enzyme, 32.6 mM 5'-IMP was synthesized from inosine with a 41% molar yield, but the synthesized 5'-IMP was hydrolyzed back to inosine and phosphate as the reaction time was extended.