Three different ligament prostheses (Leeds-Keio, Gore-Tex and a prototype of Aramid) were implanted in sheep knee joints replacing the anterior cruciate ligament and the medial collateral ligament. After 1 yr the knees were explanted and their biomechanical properties were assessed by a drawer test and tensile tests of the implants. Additionally the ligament replacements, the synovial membrane and the lymph nodes were inspected histologically. For each type of prosthesis partial or total ruptures occurred. None of the operated joints regained normal stability and stiffness. Anterior knee stability was best for the Gore-Tex treated group and worst for those joints that received a Leeds-Keio implant. The stiffness and rupture strength were highest for the Gore-Tex prostheses. Histologically the strongest intra-articular inflammatory response was observed in the Gore-Tex treated joints. Inside the drill tunnels the severest foreign body reaction was found for the Aramid prostheses. Gore-Tex fibres often showed good bony integration. Wear particles formed from all prostheses and caused small granulomas and mild synovities. The results suggest that none of the tested materials represent an ideal solution for ligament replacement. Relatively good stability does not guarantee good biocompatibility and vice versa. This suggests that ligament prostheses should be applied only in salvage cases.