Several reports have claimed that silicone gel (SG) or oil (SO) in breast implants may act as adjuvants to mediate an autoimmune response to connective tissue proteins (CTP). In this study, rat models were used to determine if antibodies could be raised against homologous CTP (collagen type I or II, or fibronectin) when SG or SO were used as adjuvants. Female Dark Agouti (D/A) rats, which are genetically susceptible to autoimmune disease, and Sprague-Dawley (S/D) rats were injected once in the mammary gland region with either 1.) CTP emulsified with SG or SO, or 2.) SG or SO without added CTP to mimic leakage from breast implants. Rats were bled before and at different times after immunization. Sera were analyzed for antibodies to CTP by ELISA. Sera from S/D rats injected with SO without added CTP showed significant levels of IgG antibodies against collagen II after 5-24 weeks (latest time point tested). Antibodies showed a dose-dependent titration, and antigen-antibody binding was inhibited specifically by collagen II. Sera from D/A rats immunized with collagen I emulsified with SG showed significant levels of IgG antibodies against collagen I after 24 weeks, with maximum levels at 52 weeks. Autoantibodies to fibronectin were not detected. These results indicate that silicone gel or oil may induce specific autoantibodies against CTP.