Implants of polyurethane-coated polyester (Dacron), polyurethane-coated PTFE (Teflon), or uncoated PTFE were used to substitute the medial rabbit meniscus, and were observed over a 3-month period. Joint mechanics were similar for sham-operation, meniscectomy, or substitution with a coated implant. Joints with an uncoated PTFE prosthesis tended to be less compliant and to show greater load dissipation during relaxation. Cartilage indentation showed softening with all alternatives compared to sham-operation. Ingrowth into the prosthesis periphery was best with the PTFE implants. Both types of coated prosthesis retained their original shape, in contrast to the uncoated prostheses. To the naked eye the cartilage appeared unaffected only after sham-operation or implantation of a coated PTFE prosthesis. Osteophyte formation was common with all alternatives except sham-operation. We conclude that replacement of the medial meniscus with coated prostheses, especially the PTFE, was better than meniscectomy or using the uncoated prosthesis. However, cartilage softening and osteophyte formation indicate that joint mechanics were not restored to normal.