Women with silicone breast implants have a significantly increased frequency of antibodies to collagen types I and II. To characterize the specificity of these antibodies, 70 women without a specific autoimmune disease, according to the criteria of the American College of Rheumatology, but who had silicone breast implants were studied for the presence of serum antibodies to native and denatured human types I and II collagen by ELISA. Positive sera were further studied by immunoblotting using peptides derived by cyanogen bromide digestion of the collagens. Samples of 82 women with systemic lupus erythematosus (SLE), 94 women with rheumatoid arthritis (RA), and 133 healthy controls were studied concurrently. There was a high frequency of autoantibodies to collagen in each of the study groups when compared to the healthy controls. However, and of particular interest, the epitope specificity of the autoantibodies differed markedly. Sera from women with silicone implants reacted strongly in an individual-specific manner with multiple peptides of type I collagen, whereas sera from women with SLE and RA reacted only weekly with a restricted range of peptides of type I collagen. Sera from women with RA reacted strongly with multiple peptides of type II, whereas sera from women with silicone implants or SLE reacted only weakly. The reactivity of women with silicone implants suggests that silicone or its biodegradation products can act as adjuvants in situ to enhance the immunogenicity of type I collagen, or protein-silicone conjugates.