Nonstructural protein 15 (Nsp15) of the severe acute respiratory syndrome coronavirus (SARS-CoV) produced in Escherichia coli has endoribonuclease activity that preferentially cleaved 5' of uridylates of RNAs. Blocking either the 5' or 3' terminus did not affect cleavage. Double- and single-stranded RNAs were both substrates for Nsp15 but with different kinetics for cleavage. Mn(2+) at 2 to 10 mM was needed for optimal endoribonuclease activity, but Mg(2+) and several other divalent metals were capable of supporting only a low level of activity. Concentrations of Mn(2+) needed for endoribonuclease activity induced significant conformation change(s) in the protein, as measured by changes in tryptophan fluorescence. A similar endoribonucleolytic activity was detected for the orthologous protein from another coronavirus, demonstrating that the endoribonuclease activity of Nsp15 may be common to coronaviruses. This work presents an initial biochemical characterization of a novel coronavirus endoribonuclease.