The membrane present at the bone-cement interface was retrieved from twenty patients with a loose, non-septic failed total hip replacement at a site clearly remote from the pseudocapsule that reformed postoperatively. The orientation of the membrane was carefully marked to identify the surface in contact with cement. The membrane was studied histologically, histochemically, by cell culture, by organ culture, and by assessment of its ability to synthesize prostaglandin E2 and collagenase. This membrane, rather than being a nondescript so-called fibrous membrane, has the histological and histochemical characteristics of a synovial-like lining. The synovial-like cells are adjacent to the cement layer. Deep to them macrophages predominate. Inflammatory cells are absent. Cell cultures of this membrane contain stellate cells similar to those found in cell cultures of normal and rheumatoid synovial tissue. This membrane has the capacity to produce large amounts of prostaglandin E2 and collagenase. Clinical Relevance: This transformation of tissue at the bone-cement interface in patients with a non-septic, loose total hip component to a synovial-like tissue with the capacity to generate prostaglandin E2 and collagenase may explain the progressive lysis of bone that is seen in some patients with loose cemented total joint implants. Loosening of the component may be a stimulus to the synthetic activity of this tissue, which leads to further resorption of bone. Understanding and the possibility of pharmacological control of this membrane may contribute to improved duration of total joint implants.