Dinucleotides containing guanine, when soaked into crystals of bovine pancreatic ribonuclease, have been found to bind in an unexpected manner, quite unlike interpretations of earlier X-ray diffraction studies. This finding has prompted a re-examination of three mononucleotide-RNase complexes from this laboratory resulting in a re-interpretation of the complex that involved a guanine mononucleotide. © 1992.