Following application of factor XIII for 3 weeks bone healing was studied in Sprague-Dawley rats on the 20th postoperatively day qualitatively (radiographs, scintigraphy) and quantitatively (determination of callus thickness, of bone mineral content, and scintillation counting). No significant difference could be detected between animals treated with factor XIII and animals which served as controls. Therefore, a stimulating effect of factor XIII upon bone healing can not be expected in case of a normal factor XIII serum-level. The application of factor XIII is indicated only in high risk patients with a low factor XIII serum-level.