Nearly all resonances were assigned in the two-dimensional1H NMR spectra of binase, guanyl-ospecific ribonuclease from Bacillus intermedius containing 109 amino acid residues. The exchange rates of amide protons with the solvent deuterium were measured in2H2O at pH 6.7 and 30°C. Coupling constants3J of H-NCα-H, NOE contacts, solvent exchange rates of amide protons, and indices of CαH chemical shifts were measured, and the binase secondary structure was deduced from these data. It involves three α-helices in the N-terminal part (the 6-16, 26-31, and 41-45 segments) and a β-sheet formed by five antiparallel β-strands (51-55, 71-75, 86-90, 95-99, and 104-108 segments). The binase secondary structure was compared with that of its closest homologue, barnase from B. amyloliquefaciens.