We studied tissue reaction to polymeric silicone (Silastic), polythylene, tantalum wire, and polytetrafluorethylene (Teflon) in temporal bones and to Silastic in removed shunt tubes. A purpose of this study was to determine which synthetic material is best for insulation of cochlear implant electrodes. We found fibrous tissue around all shunt tubes and in all bones, giant cells in some specimens, and lymphocytic infiltration in a few.We consider none of these findings to indicate bioincompatibility, except in one case with a presumed idiosyncratic reaction to Silastic. We conclude that all four materials are well tolerated by the human ear. Our finding that Silastic is well tolerated for long periods of time leads us to conclude that it is a good choice for insulation for implanted wires.