The aminoterminal 1 -34 sequence of human parathyroid hormone (HPTH) has been synthesized by the solid-phase procedure. The synthetic peptide was found to be biologically active in both the /// vitro rat renal adenylate cyclase assay (1030 U/mg) and the in vivo chick hypercalcemic assay (7400 U/mg). The lower potency of the synthetic human 1-34 peptide in the adenylate cyclase assay compared to the corresponding synthetic bovine 1 -34 parathyroid hormone (5400 U/mg) most probably arises because of the presence of serine rather than alanine at position l in t he sequence. The synthetic analogue [Ala 1 ]HPTH-(l-34) was prepared and found to be significantly more active in the adenylate cyclase assay (4085 U/mg), confirming the importance of the Ala 1 position in the renal in vitro assay. The human and bovine 1-34 peptides were equipotent in the chick hypercalcemia assay. Immunological studies showed that on a molar basis the synthetic human 1-34 peptide and the native HPTH-(1 -84) were equipotent in displacing 125