Osteocalcin (bone gla protein) is a sensitive marker of bone turnover in metabolic disease. Using a well characterized antiserum (R 102M) we have assayed serum and synovial fluid samples from patients with osteoarthritis (OA) and rheumatoid arthritis (RA) and related levels to serological and radiological markers of disease. There were 21 patients with RA (mean age 58.2 years, 15 F) and 33 with OA (mean age 69.2, 28 F). Paired serum and synovial fluids (SF) were available in 19 RA patients and 30 OA patients. Serum osteocalcin levels were related to age-/sex-matched normals and to a small group of elderly disease controls. Serum levels tended to be lower in RA than controls, but not significantly so: RA 5.56 (3.67); control 6.09 (2.54) ng/ml; expressed as mean (SD) and the mean serum/SF ratio was 0.88 (0.86). The results were much more variable in OA (mean serum osteocalcin 6.1 (3.9]. Elevated levels were mainly due to a small number of patients with a destructive form of OA and were higher than those with non-destructive OA (10.3 (3.5), n = 20, versus 3.83 (1.6), n = 10). Patients with non-destructive OA had a lower serum osteocalcin than age-/sex-matched normals. In this study, synovial fluid levels were usually less than serum concentrations, but in two RA and four OA patients the ratio was reversed, suggesting local production. Osteocalcin may be an important marker of bone activity in OA.