A 5.5 Å electron density map of ribonuclease has been obtained by X-ray diffraction using five isomorphous derivatives. One of the cystine residues is identified by means of a chemically modified ...

A model is proposed of the polypeptide chain in bovine pancreatic ribonuclease based on a 2 Å electron density map involving 7,294 reflexions and data from seven heavy atom derivatives.