The present investigation has shown that crude papain can be used to produce rapid changes in the epiphysial cartilage of various young laboratory animals (rabbits, mice, rats, guinea pigs and cats).

1. Single injections of crude papain produce profound changes in the epiphysial cartilage. These changes disappear within a few days. They are radiographically visible as a narrowing of the epiphysial plates. Histologically, the formation of bony trabeculae in the primary spongiosa is found to be arrested.

2. Repeated injections of crude papain cause permanent damage to the epiphysial cartilage, often with bony closure. Consequently, the longitudinal growth of the injected animals, when compared to the controls, is found to be retarded or permanently arrested, and there may be severe bony deformity.

3. Using inactivated crystalline papain, we have been able to produce changes in the epiphysial cartilage identical with those caused by the injection of crude papain.

4. The injection of crude papain is dispelled by the addition of cysteine, but retains its full strength if hydrogen peroxide is added.