A new monoclinic form of bovine pancreatic ribonuclease A was obtained together with the known trigonal form using sodium chloride and ammonium sulfate as precipitants. The new monoclinic form was stable at high salt concentrations. Crystal structure analysis of the new monoclinic form was carried out, and the intermolecular interactions were compared with those of the trigonal form. In the new monoclinic form, there were two pairs of non-crystallographic dimers, both of which were almost equivalent to the dimer in the trigonal form. Molecular layers were formed by the dimers, and a water region was spread between the molecular layers. Although the overall crystal structures of the two crystal forms are quite different, common successive interaction between the dimers aligned along the two-fold screw axis. Analyses of the contact surfaces were carried out, together with the other crystal forms of native ribonuclease A, resulting in a qualitative understanding of the relationship between intermolecular interactions and the crystallization conditions. (C) 2011 Elsevier B.V. All rights reserved.