1. Immunological assay systems specific to the amino-terminal region of parathyroid hormone were used to study the properties of human parathyroid hormone (HPTH 1-84) and of synthetic peptides. One of these peptides (HPTH 1-34A) had the sequence proposed by Niall, Sauer, Jacobs, Keutmann, Segre, O'Riordan, Aurbach & Potts (1974). The others (HPTH 1-34B and C, synthesized by different methods) had the sequence proposed by Brewer, Fairwell, Ronan, Sizemore & Arnaud (1972).
2. HPTH 1-84 and HPTH 1-34A behaved identically in a number of these systems. In some systems, especially those using antisera obtained by immunizing with HPTH 1-34A, differences were, however, found. These may be due to conformational or other physical differences or to the presence of an immunologically incomplete site in the fragment.
3. In systems in which HPTH 1–84 and HPTH 1-34A behaved identically the peptides with the sequence proposed by Brewer et al. (1972) were of low reactivity by 100–2000-fold.