The effect of tobacco tars on bronchial mucosa and the relationship of such products to bronchogenic cancer continue to be subjects of considerable interest and investigation. The increasing incidence of bronchogenic carcinoma and the statistical evidence showing a correlation with tobacco consumption demonstrate the need for continued work in this field. The studies here being reported will demonstrate a different technique for the evaluation of the effects of crude tobacco tar on regenerating tracheal epithelium as well as report the results obtained. Methods and Materials In this study a 2 cm. square area of the anterior lower cervical trachea was excised. Ivalon sponge, compressed to one-fourth its usual thickness, was substituted for the defect thus created. In 36 dogs, sterile, chemically clean Ivalon prostheses were substituted for a portion of the anterior wall of the trachea. These dogs were killed at intervals of 1, 2, 4, and 12 weeks to