The changes of bone gamma-carboxyglutamic acid-containing protein (BGP) levels in bone and serum were studied in relation to those in calcium metabolism using chick embryos and chicks aged from 13 days' incubation to 8 weeks old. Chick BGP was determined by radioimmunoassay using antiserum to purified chick BGP. BGP levels in bone and serum increased significantly at hatching and then decreased until 3-5 days of age. Thereafter, BGP levels in bone and serum increased gradually until 8 weeks of age. These changes of BGP levels were well correlated with those of serum calcium and inorganic phosphorus concentrations, serum alkaline phosphatase activity and bone calcium content. The molecular size of increased serum BGP at hatching was not different from that of bone BGP. These results suggest that BGP plays a role in hatching and bone formation during chick development.