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1. Protein-depleted rats are resistant to the lethal effects of carbon tetrachloride. The LD(50) is 6.4ml./kg. in stock rats and 14.7ml./kg. in rats fed on protein-free diets. 2. Protein-depleted rats are resistant to carbon tetrachloride in its effect on the liver as judged by histology, accumulation of liver water, and plasma enzyme and bilirubin measurement. 3. The protection is present after feeding rats on a no-protein diet for 4 days. It is present after feeding rats on a 3%-casein diet, and partly found after feeding rats on a 6%-casein diet. 4. The activities of the microsomal enzymes that demethylate Pyramidon and hydroxylate benzopyrene in the liver fall by over 80% in rats fed on the no-protein diet for 4 days or more, or in rats fed on a 3%-casein diet. A 50% fall is found in rats fed on a 6%-casein diet. 5. A single dose of DDT or three doses of phenobarbitone cause increased microsomal enzyme activity in protein-depleted rats. 6. The animals are then sensitive to the lethal and liver-damaging effects of carbon tetrachloride. 7. DDT dosage also leads to increased sensitivity to carbon tetrachloride in rats fed on stock diets. 8. These findings support the hypothesis that carbon tetrachloride is metabolized by microsomal enzymes to form the true toxic compound.

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