We developed a sensitive two-site sandwich ELISA for quantitative analysis of human osteocalcin in serum or plasma. Our method is based on two different highly specific antibodies recognizing epitopes at different ends of the protein so that only intact osteocalcin is detected. The method is fast (total analysis time less than 6 h/96 wells), precise (intraassay variation less than 2.3% at four different levels; n = 10, and interassay variation less than 2.5%, n = 5, respectively), and accurate, with a mean recovery of 105%. The detection limit in serum is approximately 0.1 micrograms/liter. The mean concentration of osteocalcin in normal serum with this assay is 3.3 micrograms/liter (SD 3.7 micrograms/liter; range 0.1-13.1 micrograms/liter; n = 41), and the reference range is 0.28-10.1 micrograms/liter (10 and 90% confidence limits). The method shows a reasonable positive linear correlation with other osteocalcin assays (Incstar, r = 0.55, p < 0.05, n = 13; Henning Oscatest, r = 0.52, p < 0.005, n = 34). A good correlation (r = 0.70, p < 0.001) between individual osteocalcin and bone-specific alkaline phosphatase serum concentrations was observed in normal subjects. We found a low or undetectable concentration of intact osteocalcin in serum of all four of our patients with acute primary hyperparathyroidism, and in all five patients with hypocalcemic secondary hyperparathyroidism, which suggests that PTH effectively inhibited the synthesis of osteocalcin in osteoblasts. The serum concentration of intact osteocalcin was elevated in two of three patients with chronic primary hyperparathyroidism.(ABSTRACT TRUNCATED AT 250 WORDS)