¹ From the Department of Radiation Biology, The University of Rochester, School of Medicine and Dentistry, Rochester, New York

1. Respiration and fermentation of yeast receiving 90,000 r of 250 kv. x-rays were studied under a variety of conditions. This dose will nearly completely inhibit growth or colony formation.

2. The apparent effects of irradiation are quite dependent on the K⁺ and H⁺ of the suspending medium. At pH 4.5 stimulatory effects were observed in KH₂PO₂ buffer and inhibition in potassium-free (T-S-T) buffer. At pH 6.5 the situation was reversed and the effects were very small (about 10 per cent). Addition of K⁺ to irradiated yeast in T-S-T buffer at pH 4.5 can completely reverse the inhibition seen.

3. Starving increases the apparent radiosensitivity of respiration and fermentation, probably by depletion of metabolite and/or electrolyte reserves.

4. Treatment with a cation exchange resin (dowex 50) results in marked inhibition of these processes in irradiated yeast, either fresh or starved. This was most effective if given after irradiation. Almost complete inhibition of anaerobic CO₂ production occurs with starvation, irradiation, and dowex treatment combined.

5. The effects of starvation and cation exchange resin treatment can be reversed, though not completely, by adding K⁺ to the medium.

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