1. The present paper describes a detailed study of the specificity of high-affinity antibodies obtained from the yolk of eggs laid by a chicken successfully immunized with synthetic human parathyroid hormone (hPTH)-(1-34). 2. Using 125I-labelled bovine parathyroid hormone (bPTH)-(1-84) purified by high performance liquid chromatography (HPLC) as tracer, and hPTH-(1-34) as reference, we found superimposable curves with hPTH-(13-34), bPTH-(13-34) and bPTH-(1-34); the [Asp-76]hPTH-(1-84) peptide showed a molar percent cross-reactivity (calculated at 50% B/BO) of 73% and the bovine sequence 1-84 of 15%. 3. Studying amidated [Tyr-34] bovine PTH fragments we found the highest cross-reactivity with the peptide 7-34 (33%) followed by 20-34 (1.7%) and 25-34 (0.4%). The bovine sequence 1-25 showed a low reactivity (0.7%) and the 1-12 sequence none at all. Rat parathyroid hormone (rPTH)-(1-34) showed low cross-reactivity (3.1%), the same occurring with peptide [Tyr-34, Norleu-8,18] bPTH-(1-34) (1.5%). 4. The data suggest that the antibodies studied recognize epitopes in or near amino acids 18 to 25 of the sequence of the hPTH molecule. This region of the molecule is coincident with the antigenic determinant predicted by the analysis of the hydrophilicity plot of the hPTH-(1-34) peptide.