The Immulite parathyroid hormone (PTH) immunochemiluminometric assay (ICMA) was evaluated by measuring serum PTH concentrations in 134 volunteers and 25 patients with various diseases affecting bone and mineral metabolism and comparing with those measured by the Nichols Allegro immunoradiometric (IRMA) assay. Although the assays were highly correlated (r = 0.99. P < 0.0001), there were discrepancies at lower PTH concentrations which may be explained by the lower limit of detection of the ICMA assay. The anti-coagulants EDTA and heparin appeared to have significant effects on PTH measurement particularly at lower concentrations. Sera assayed by ICMA diluted out more uniformly than in the IRMA and there was no cross reactivity with parathyroid hormone-related peptide in either the IRMA or ICMA. Delays of up to 8 h in specimen handling did not appear to affect PTH measurements. The enhanced sensitivity of the Immulite ICMA intact PTH assay confers an advantage in the diagnosis of both hyper- and hypocalcaemia, while the shorter incubation times, automation and lack of radioactivity are important practical advantages.