Osteocalcin or bone gamma-glutamic acid-containing protein (GLA protein) was isolated from human bone and used to develop a homologous radioimmunoassay of human osteocalcin. The effect of age on serum osteocalcin was studied in 380 normal children and adolescents and 330 normal adults. The mean (+/- SD) values in adults were higher in men [25 +/- 5 micrograms/L (4.3 +/- 0.8 nmol/L)] than in premenopausal women [20 +/- 6 micrograms/L (3.4 +/- 1.0 nmol/L); P < 0.01], but both were lower than in postmenopausal women [29 +/- 2 micrograms/L (5.0 nmol/L)]. The highest concentrations were seen in girls [ages 10-12 years: 99 +/- 38 micrograms/L (17.0 nmol/L)] and boys [ages 14-16 years: 107 +/- 57 micrograms/L (18.4 nmol/L)]. These mean values were substantially higher than those previously reported for results of heterologous osteocalcin radioimmunoassays but the correlation (r = 0.87, n = 77, P < 0.001) between both sets of results was excellent. In patients with metabolic bone diseases characterized by high or low bone turnover, the increase or decrease in serum osteocalcin observed was as expected. This homologous radioimmunoassay of human osteocalcin thus reflects bone turnover but reports serum concentrations higher than previously suspected.