g amma-Carboxyglutamic acid-containing protein of bone (BGP) is an abundant noncollagenous protein of mammalian bone. BGP has a molecular weight of 5,800 and contains three residues of the vitamin K-dependent amino acid, gamma-carboxyglutamic acid. We have applied a radioimmunoassay based on calf BGP for the measurement of the protein in the plasma of 109 normal humans and 112 patients with various bone diseases. BGP in human plasma was demonstrated to be indistinguishable from calf BGP by assay dilution studies and gel permeation chromatography. The mean (+/- SE) concentration of BGP in normal subjects was 6.78 (+/- 0.20) ng/ml, 7.89 (+/- 0.32) for males and 4.85 (+/- 0.35) for females. Plasma BGP was increased in patients with Paget's disease of bone, bone metastases, primary hyperparathyroidism, renal osteodystrophy, and osteopenia. Plasma BGP did correlate with plasma alkaline phosphatase (AP) in some instances, but there were dissociations between the two. It was additionally observed that patients with liver disease had normal plasma BGP despite increased plasma AP, a reflection of the lack of specificity of AP measurements for bone disease. Our studies indicate that the radioimmunoassay of plasma BGP can be a useful and specific procedure for evaluating the patient with bone disease.