Allogeneic and xenogeneic biological tendon and ligament materials for ligament replacement were tested in the knee of rabbits and sheep. In-vitro and ex-vivo biomechanical tests as well as micromorphological investigations following implantation were performed. The preparation procedure mainly consists in cross linking of collagen by dicarboxylic acids (DC). Modifications in the method induce changes in the biomechanical stability also after implantation. Sheep with autologous patellar tendons substitutions and glutaraldehyde (GA) fixed bovine tendons served as controls. Because of biocompatibility reasons the later seem to be not yet suitable for clinical applications. The DC-tendons need for improved midterm stability prior to clinical implantations; the biocompatibility appears to be excellent.