1. The radioprotective activity of several hundreds of fermentation liquids have been examined by methods described in our previous reports. 2. Out of the tested fermentation liquids, good protective ability was rendered-mainly after acid hydrolysis-by the fermentation liquid of Penicillium notatum strain which is applied also in industry. 3. The agent, when it was produced, proved to be d-Penicillamine. 4. The radioprotective activity of d-Penicillamine as well as its ability to reduce the toxicity of nitrogen mustard have been compared with the effect of other wellknown radioprotectors, on an equimolar basis. 5. It was found that its protective ability for E. coli B against both X-ray and nitrogen mustard is about the same degree as that of cysteine. 6. From the results, theoretical and practical conclusions have been drawn.
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