The mechanical strength of trabecular bone at the ankle joint was studied in ten amputation specimens by means of multiple penetration tests. Tibial bone was considerably weaker than talar bone (40% on average). An area of peak bone strength located posteriorly and medially to the center of the joint was found in most joints. The variation of bone strength with the depth from the surface was studied by comparing five successive 2-mm levels. At the tibia there was a statistically significant decrease of bone strength from the first to the second level. At the talus, bone strength increased at first, then decreased significantly. The absolute values of strength recorded suggest that in a substantial number of the joints studied, the resection surfaces--especially at the tibial side--might be too weak to support the loads imposed by current prosthetic designs.