A nucleoside phosphotransferase has been purified to homogeneity from barley seedlings. Its Mr is about 50,000 and it consists of two subunits of equal size. A tightly bound metal ion required for activity can be replaced by Mg, Ba, or Co ions, but not by divalent Cu, Ca, Cd, or Hg. The enzyme is capable of catalyzing the transfer of phosphate from nucleoside monophosphates to the 5'-hydroxyl of any other nucleoside, but shows a decided preference for purine deoxynucleoside phosphate acceptors. A short lived phosphoryl enzyme intermediate has been trapped by rapid denaturation in presence of [32P]AMP. The steady state kinetics of this enzyme is found to be fully consistent with a branched ping pong pathway involving a compulsory phosphoryl enzyme intermediate from which phosphate may be transferred to nucleoside or to water. With appropriately chosen substrates, parallel line reciprocal plots were obtained, provided that both products of the branched pathway were taken into account in the rate measurements. Conversely, when only transphosphorylation was measured, converging reciprocal plots were obtained. Accordingly, assays were devised for the measurement of three velocity components: transphosphorylation, hydrolysis, and the sum of both reactions. Competitive inhibition of the transferase reaction by higher concentrations of either substrate is attributed to deadend interactions. Instead of being inhibited by the formation of a putative E-P.AMP complex, however, the phosphatase reaction is accelerated almost 2-fold.