The influence of calcitonine, amino acids, ribonucleid acids, kallikrein, growth hormone, and cyclic adenosine 3'5' monophosphate on fracture repair was tested in rabbits. To obtain an objective measurement of callus stability the bones were bent mechanically. The force applied and the degree of bending were recorded continuously. The slope of the curve was taken as a parameter of callus stability. In the series with calcitonine no influence on fracture healing was found. Injections of ribonucleic acids, amino acids, kallidrein, and growth hormone demonstrated a limited effect on bone repair. Callus formation was stimulated by adenosine 3'5' monophosphate. Optimal results were achieved by a combination of adenosine 3'5' monophosphate, amino acids, ribonucleic acids, and kallikrein.