The ability of the serine protease subtilisin BPN’ to catalyze peptide bond formation between fragments containing noncoded amino acids, peptide mimetics, and peptide conjugates in a kinetic approach was explored. It was found that the enzyme accepts numerous of these types of compounds both as acyl donor and acyl acceptor. The results together with specificity studies reported by others provide an active site model as a guideline in the design of enzymatic synthesis of biologically important compounds.