The biocompatibility of silicone implants has been a source of long-standing controversy. An attempt to ascertain this biocompatibility with regard to the cellular immune mechanism was undertaken. Silicone gel from a mammary prosthesis was sonicated with Freund adjuvant under aseptic conditions to produce an injectable mixture, and injected subcutaneously in 10 female guinea pigs. Control animals did not receive this initial stimulus. After four weeks each animal received a challenge silicone implant of the same product in a gel form, surgically placed intraperitoneally in the omentum. Four weeks later peritoneal exudate cells were harvested for migration inhibition studies. Site of implantation and regional lymph nodes were processed for histological and electron microscopy.The migration inhibition studies showed that the silicone environment resulted in 45% inhibition of migration of the sensitized cell population. All sites of implantation showed a marked inflammatory response with silicone inclusions in giant cells, surrounded by neutrophils, plasma cells, and lymphocytes. Electron microscopy and x-ray energy spectrograph (XES) analysis revealed cytoplasmic transference of silicone from macrophages to lymphocytes, via a cytoplasmic bridge. There was no comparable response in control animals. Although apparently inert, silicone is capable of eliciting a cellular immune response demonstrated by the migration inhibition technique. This response is comparable to that elicited by purified protein derivative and may indicate that silicone acts as a hapten-like incomplete antigen.