The measurement in blood of alkaline phosphatase is an important adjunt to imaging procedures for clinical assesment of skeleton. In the present study the relationship between serum alkaline phosphatase activity and bone formation was examined following the clinical use of Pyrost material. This bone formation marker was measured together with serum calcium and phosphate in 12 patients after the implantation of Pyrost into surgical bony defects. Blood samples were obtained before the surgical procedure which served as control, and postoperatively at 1,7,15 and 30 days. No correlation between the levels of serum alkaline phosphatase, calcium and phosphate and bone formation was observed.