In this review article, current thinking about the infection caused by Borrelia burgdorferi is presented. The problems associated with diagnosis and treatment of Borrelia burgdorferi infection are pointed out. A series of monoclonal antibodies recognizing Borrelia burgdorferi-associated antigens is presented. In collaborative research projects these antibodies have been 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 for diagnosis and therapy of the Borrelia burgdorferi infection. In this second part of the review article the above-mentioned molecular biological and protection experiments are summarized.