An homologous radioimmunoassay for the synthetic 1-34 amino terminal fragment of human parathyroid hormone (hPTH) was developed using antibodies obtained from yolk of eggs layed by an immunized chicken. The 125I labelled 1-34 hPTH peptide was purified by cation-exchange chromatography which provided a highly stable preparation. The specificity of the assay showed a cross-reactivity of 50% with the 1-34 hPTH (code 81/574) preparation and 27% with the 1-84 hPTH (code 79/500) preparation, when compared with the 1-34 hPTH from Bachem that was used for immunization and labelling. The minimal detectable dose of the assay was 10 pmol/l; in 69 healthy controls the values obtained ranged from less than 10 to 28 pmol/l and in 14 patients with surgically proven primary hyperparathyroidism from 10 to 519 pmol/l.