Parathyroid hormone (PTH) is a linear peptide of 84 amino acids that is found in serum mainly in the form of carboxyl-terminal fragments. The biological activity of PTH depends on the presence of the amino-terminal portion and in circulation is limited to the intact molecule. We describe an immunofluorometric assay for the measurement of PTH-(1-84) based on a chicken egg yolk-derived amino-terminal antibody bound to microtiter plates by an anti-chicken Ig monoclonal antibody. As tracer antibody we employed a Europium-labelled carboxyl-terminal specific monoclonal antibody produced from a mouse immunized with hPTH-(53-84)-BSA conjugate. The assay included an initial overnight incubation of the sample and the solid phase-bound amino-terminal antibody, followed by washing and addition of the tracer antibody, and an additional two hours of incubation prior to fluorescence reading. The least-detectable dose was in the order of 2.5 pg/ml and preliminary studies in 40 normal adults showed values in the range of 4 to 70 pg/ml; for 12 patients with surgery-proven primary hyperparathyroidism values ranged from 109 to 743 pg/ml and for 34 patients with humoral hypercalcemia of malignancy from 2.5 to 66 pg/ml. We conclude that this assay, with its increased sensitivity and specificity, will be a valuable tool in the study of PTH secretion in normal and pathological situations.