The S‐peptide of the enzyme bovine pancreatic ribonuclease has been used as a model for covalent semisynthesis. Methods for side‐chain protection, enzymatic cleavage of the peptide chain at the level of the single arginine‐10 and for selective deprotection of the α‐carboxyl function of this residue, have been examined. The partially protected [1–10] sequence has been coupled to a solid‐phase generated [11–15]sequence attached to the polymer. After deblocking from the solid‐support, the [1–15] semisynthetic peptide was complexed with native S‐protein to give a complex with high