As a new test system for therapeutic studies on lung cancer, a semi-quantitative method for formation of tumorous nodules in rabbit lungs was developed utilizing the malignant cells of V × 2 carcinoma. Pre-determined numbers of V × 2 carcinoma cells were inoculated intravenously into the test animals. A correlationship between the number of cells inoculated and the number of tumor nodules detectable on lung surface was observed to a certain extent. The possible applications and advantages of this test system was also discussed.