Anterior cruciate bone-ligament-bone preparations of wild primates tested in tension failed at a higher load and greater elongation, and absorbed more energy, at a fast rate of deformation than at a slow rate. The major mode of specimen failure changed from a predominance of tibial avulsion fractures at the slow rate to ligament disruption at the fast rate. Failure at the two deformation rates commonly occurred by a serial mechanism involving from one to all components of the bone-ligament-bone unit to different degrees. Factors important in ligament-failure properties included: (1) rate-dependent behaviors, (2) geometrical arrangement of collagen fibers, (3) surrounding ground-substance effects, (4) osseous insertion-site effects, and (5) the influence of non-physiological experimental loading conditions and disuse-induced changes in specimens.