The aim of this study was to elucidate the existence of autoantibodies directed to type I collagen as a response to connective tissue alterations that occur in inflammatory diseases. We have demonstrated the presence of antibodies to type I collagen during the time course of adjuvant arthritis, by means of the ELISA technique. These autoantibodies reached the highest levels in advanced phases of the arthritic process from 8 to 10 weeks after mycobacteria administration. The hind-paw swelling and the anti-type I collagen antibodies remained for at least 18 weeks post-induction.