We tested the anterior-posterior motion of nine normal cadaver knees in zero to 90 degrees of flexion using a specially designed apparatus. This apparatus applied a dynamic anterior-posterior force to each knee and measured the resulting tibial displacement, rotation, and torque. In the intact knee, an anterior force produced an internal tibial torque and internal tibial rotation, while a posterior force produced an external torque and external rotation. Anterior-posterior displacement increased by 30 per cent when the tibia was allowed to rotate freely about its neutral rotation position. Isolated section of the anterior cruciate ligament produced more than double the amount of anterior displacement without affecting posterior displacement. Isolated section of the posterior cruciate ligament produced almost triple the amount of posterior displacement without affecting anterior displacement. After cutting either the anterior or the posterior cruciate ligament, the resulting internal or external secondary tibial rotation disappeared. It appears, therefore, that the anterior and posterior cruciate ligaments are the primary restraints to motion in the anterior and posterior directions as well as the causes of internal and external tibial rotation during anterior and posterior motion.