A process for the recombinant preparation in a bacterial host of the mature form of a mammalian protein or peptide of formula X-Pro-Z, which is subject to processing by endogenous bacterial aminopeptidases is provided. In the formula X-Pro-Z, X is a single N-terminal amino acid other than proline, and Z is the remaining sequence of amino acid residues of the protein or peptide. The process comprises inserting into a cell of the bacterial host an appropriate vector containing DNA coding for Met-Y-X-Pro-Z, in which Y is a natural amino acid that is specifically cleavable in vitro from X-Pro-Z by an aminopeptidase and that imparts resistance to in vivo processing by endogenous bacterial aminopeptidases. The cell is then induced to express the expression product Met-Y-X-Pro-Z which is then treated with an appropriate aminopeptidase to cleave off Met and Y.