Distinction between the two major complications of total hip replacement surgery, septic bacterial culture-positive and aseptic bacterial culture-negative osteolysis and loosening, is difficult due to the eventual role of bacterial remnants and biofilms, which are recognized by cells provided by toll-like receptors (TLRs) of the innate immune system. It was hypothesized that cell typing and TLR mapping might provide new information on the pathomechanisms of loosening. To test this hypothesis, septic (n = 10) and aseptic (n = 5) interface tissue as well as mildly inflamed osteoarthritic synovial membrane (n = 5) samples were characterized and compared using antibodies against several cell line-specific markers, including fibroblast, monocyte/macrophage, T cell, B cell, plasma cell and neutrophil markers, and TLRs. In osteoarthritic synovium, TLR-positive cells were restricted to surface tissues and only few inflammatory cells were detected, whereas aseptic interface was heavily infiltrated with monocyte/macrophages, which were also the major TLR-positive cell type rendering the tissue reactive to TLR ligands. Interestingly, septic cases contained also neutrophil and lymphocyte infiltrates of which especially B-cell infiltrates might be clinically useful in discriminating the two major complications of the joint replacement surgery. (c) 2010 Wiley Periodicals, Inc. J Biomed Mater Res, 2010.