Pig cruciate ligaments were dissected and examined radiologically to evaluate their function. The variations in distance between the origin and insertion of the fibre bundles were measured in different joint positions; the maximal decrease in distance amounted to 50% in the anterior (ACL) and 30% in the posterior cruciate ligament (PCL). The ACL consists of 2 morphologically distinct portions. Most of its fibres are found to be taut in extension. Its posterolateral portion contains a 'guiding bundle' taut in all joint positions as well as fibres that are taut either in extreme positions (in extension or in flexion or in both) or in intermediate positions. Its anteromedial portion is mainly composed of fibres taut in extension with some additional fibres taut in intermediate positions. The PCL is not subdivided. Most of its fibres are taut in flexion and some in intermediate positions. Its guiding bundle is consistently taut in all joint positions. The guiding bundle of the ACL and the one of the PCL form a 4-bar link. The functions of the cruciates basically are the same in humans and in pigs, namely in guiding the joint and in restricting motion. It must, however, be borne in mind that the maximal extension of the pig corresponds to a human flexion of 30 degrees. It is not possible to correlate the differing gaits of the pig and human with respect to the function of the cruciates, as the knee has to perform flexion and extension (with secondary rotational) movements in both species, irrespective of unguligrady, digitigrady or plantigrady.