Results

About 50% of all the rats died spontaneously, usually from pneumonia, during this study. There was no significant difference between the mortality rate of the papain-treated group and that of the sum total of all the other intra-tracheally treated animals or that of any treated group taken separately. For the first 7% months, there was a steady almost linear and identical increase in mortality (to 25%) in all groups in-clusive of the dust control. During the next 1% months, all intratracheally treated groups suffered a steep increase in mortality to 50%, whereas the mortality of the control group con-tinued its linear rise to 35% during the next 24 months.

The lungs of all rats injected intratracheally with papain developed emphysema of centrilobular type, even those of an animal that died spontaneously shortly after the third intra-. tracheal injection (total dosage, 6 mg) 14 weeks after the first injection, whereas no emphysema was found in animals of the other groups. The earliest manifestations of emphy-