In contrast with kidney, heart and liver transplantations, lung transplantation involves an organ which has contact with the environment and bears a special risk of infection. This circumstance and the diminished resistance during immunosuppressive therapy makes long-term treatment with an effective broad-spectrum antibiotic advisable for lung transplantations. Taking into consideration the different objectives and questions within the research project concerned the 182 lung allotransplantations carried out in dogs can be divided into two therapeutic groups in regard to postoperative treatment: lung allotransplantations with and without immunosuppressive therapy (methylprednisolone, azathioprine). Long-term doxycycline therapy was carried out in both groups. Those animals which were not treated with immunosuppressive drugs survived for an average of 9 days, those receiving immunosuppressive therapy survived for an average of 30 days. The longest survival period was 2.5 years. Clinically and histologically it could be demonstrated that doxycycline was successful both during the immediate postoperative period and in antibiotic long-term therapy after experimental lung transplantations.