Animal experiments at the Nat. Cancer Inst. have established that only intra-tumorous application of BCG vaccine has a lasting immunotherapeutic effect on transplantable tumours. However, clinical observations have proved that BCG immunotherapy by the intra-tumorous route increases the incidence of complications, such as allergic reactions and generalized spread of BCG, to an unacceptable level. Ribi has produced a vaccine from mycobacterial fractions which is low in protein and which, as proved in clinical pilot studies in cases of melanoma and cancer of the breast, is so well tolerated that it can be injected into the tumour. Since bronchogenic carcinoma is in the majority of cases inoperable, but, on the other hand, can be reached directly via the bronchoscope or the perthoracic route a pilot study with the Ribi vaccine was started in patients with lung cancer. The preliminary results are reported.