The precursor of bovine proparathyroid hormone was synthesized by translation of parathyroid mRNA in a wheat-germ cell-free system. The amino acid sequence of the NH2-terminal extension (the pre sequence) was determined by repetitive Edman degradation of the polypeptide labeled with radioactive amino acids (radiosequencing). The pre sequence of pre-proparathyroid hormone is (formula: see text) which is followed by the sequence of proparathyroid hormone. It is significant that 20 of the 25 amino acids in the sequence are hydrophobic. This high hydrophobicity is consistent with the proposed role of the pre sequence as a membrane-penetrating peptide. The precursor-specific sequence of 31 amino acids was snythesized chemically by the solid-phase technique. This synthetic peptide was shown to bind to the microsomal fraction of homogenates prepared from extracts of parathyroid glands, a finding consistent with the proposed role of the precursor peptide in the attachment of the nascent chain--mRNA--ribosome complex to the endoplasmic reticulum.