Sequence determination of up to 24 amino-terminal residues of several putative precursors for dog pancreas secretory proteins, synthesized in vitro by translation of their mRNA's in the presence of radioactively labeled amino acids, revealed extensive sequence homology in the 16 amino-terminal residues. It is suggested that this common sequence constitutes a metabolically short-lived peptide extension which precedes the amino-terminal sequences of all pancreatic secretory proteins and that it functions in the transfer of these proteins across the microsomal membrane. This sequence was found to contain an unusually large percentage of hydrophobic residues.