The radiotherapeutic test may be defined as a diagnostic procedure in which the histologic family to which a lesion belongs is determined by its response to a measured quantity of roentgen rays. Desjardins (4) was one of the more enthusiastic proponents of the radiotherapeutic test. It was his belief that it is based on the well established fact that each variety of cell in the body has a specific sensitiveness, or a specific range of sensitiveness, to roentgen rays and radium. Because of this, he felt that the radiologist could distinguish absolutely between lymphoma and any variety of epithelial or connective-tissue tumor. Desjardins published his opinion in 1933, which was the year in which the first pneumonectomy was performed for carcinoma of the lung. At that time thoracotomy was not used as a diagnostic procedure, because of the high morbidity and mortality rate. It was therefore necessary to resort to every possible method of diagnosis before thoracotomy was considered. Because of this, the radiotherapeutic test was frequently of value. Since 1933, advances in thoracic surgery have come rapidly. Thoracotomy is no longer considered dangerous, and it is frequently used to establish the diagnosis when simpler methods fail. This progress has dampened the enthusiasm for the radiotherapeutic method. The test is still being used, however, and as recently as 1946 it was recommended that it should precede surgery in every diagnostically doubtful case (3). At the present time there is seldom any reason to recommend a radiotherapeutic test preoperatively. If the diagnosis cannot be established by any of the conventional methods of diagnosis— bronchoscopic examination and biopsy, microscopic examination of bronchial washings, needle biopsy, or biopsy of an existing metastatic focus—thoracotomy is the next step unless contraindicated. There are two reasons for avoiding the radiotherapeutic test: 1. When it is employed to make a diagnosis, operation may be delayed long enough to cause an operable lesion to become inoperable. 2. Because of the apparent variability in the response of lesions of similar histology to similar doses of x-rays, the test is frequently inaccurate. The inaccuracy of this method has been pointed out by other writers in the past, but it became more apparent to the members of our department when we reviewed all of the intrathoracic lesions which we had treated in a two-year period. Of 60 cases of malignant intrathoracic mass lesions treated, 23 were suitable for evaluation. They were histologically diagnosed, were easily visualized on roentgenograms, and had received enough radiation to produce a change in the size of the lesion. From these 23 cases, the following have been selected to demonstrate the variable response of histologically similar and dissimilar lesions to comparable radiation doses. All of the patients were treated by a 400-kv constant-potential generator.