Provided is a method for characterising a polypeptide or a population of polypeptides, which method comprises the steps of: (a) contacting a sample comprising one or more polypeptides with a lysine reactive agent to cap ε-amino groups; (b) optionally reacting the sample of polypeptides with an amine reactive reagent to block α-amino groups; (c) digesting the sample of polypeptides with a cleavage reagent to produce peptide fragments; (d) optionally deactivating the cleavage reagent; (e) removing those peptides having uncapped or unblocked amino groups; and (f) recovering the N-terminal peptides.