In this review article the current concepts on the infection caused by Borrelia burgdorferi are presented. The problems associated with the diagnosis and therapy of the Borrelia burgdorferi infection are pointed out. A series of monoclonal antibodies recognizing Borrelia burgdorferi-associated antigens is presented. In collaborative research projects these antibodies were used for molecular biological (cloning and sequencing of Borrelia burgdorferi antigens) and immunological (protection experiments by passive transfer of monoclonal antibodies in an animal model) experiments. Taken together, these studies disclosed new aspects of the diagnosis and therapy of Borrelia burgdorferi infection. In the first part of the review article the immunochemical characterization of the aforementioned antibodies and their use for antigen detection by immunohistochemistry is described.