1. The inactivating effect of soft x-rays on trypsin in solutions of various degrees of concentration has been studied.

2. It has been found to run parallel with spontaneous heat inactivation. It is almost, if not entirely, confined to the free or active trypsin.

3. Under radiation of constant intensity, the inactivation follows the simple exponential law which indicates a monomolecular reaction.

4. Estimates have been made of the amount of ionization required to inactivate trypsin to half value in these experiments and in those of Hussey and Thompson, who employed the beta rays from Radium B and C. The close agreement corroborates the idea that the effect is a function of ionization only.

5. The nature of the process of inactivation is discussed; inactivation seems to result from electrical neutralization of the trypsin ion.

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