To measure the strength of the juxta-articular trabecular bone in knee replacement, an osteopenetrometer was developed. The principle of its function is tha a needle penetrates the bony tissue and during this penetration the pressure on and the displacement of the needle is registered. The instrument was tested on cadaveric specimens of the proximal tibia. Good agreement was found between the osteopenetrometer findings in 18 cases and compression tests on the same bony structure. Forty-five measurements at different sites of the tibial resection surface revealed that the force needed for the penetration varied according to a pattern known from the determination of the elasticity modulus and ultimate compressive strength of bone in the same area. Histological examination of the penetration canals in the bone disclosed only sparse upsetting of bone. Curves from corresponding bone proved to reflect, in height as well as shape, the morphology of the skeletal structure. The osteopenetrometer appears to be applicable fo objectively assessing the solidity of trabecular bone during the operation. Thereby it is possible, in each individual case, to form a basis for more varied planning of the postoperative treatment programme.