Two children, 4 and 8 years old, were skin tested with sensitins and found to have indurations of 20 mm. Sensitins are tuberculins deriving from non-tuberculous mycobacteria. The children showed pathological enlargement of mediastinum on chest radiography. They were further examined, and no findings supported malignancy nor tuberculosis. The chest radiographs were normalizing within a few months. The large sensitin reactions together with the other findings indicate that the children had been infected with non-tuberculous mycobacteria.