1. The inactivation by γ-rays of tobacco mosaic virus is studied at various concentrations. It is found that the inactivation dose is independent of concentration at high concentrations, and at low concentrations also attains a constant, but lower, value. Over an intermediate range the inactivation dose increases with increase of concentration.
2. These facts are explained on the basis that when irradiated dry or in concentrated solution the inactivation is direct and due to ionization produced inside the virus particle. At lower concentrations the inactivation is largely indirect and due to ionization of the water.
3. Gelatin added to the solution protects the virus against the indirect action of radiation.
4. Curves are given of the inactivation of dry preparations of ribonuclease and adenylpyro-phosphatase (myosin) by X-rays.
5. It is shown that on the assumption that a single ionization in an enzyme molecule leads to its inactivation, measurement of the inactivation dose leads to a rough estimate of the molecular weight of the enzyme.
6. There appears to be no fundamental difference in the mechanism of radiation-inactivation of viruses and enzymes.
All irradiations were carried out at the Strangeways Laboratory. The virus was prepared and tested at the Plant Virus Station and the Molteno Institute. The enzymes were prepared and estimated at the Biochemical Laboratory. We are indebted to the British Empire Cancer Campaign for defraying the cost of the X-ray equipment at the Strangeways Laboratory.