Albino rats fed for 28 days from weaning on diets containing progressively smaller percentages of casein become progressively more susceptible to the lethal effects of single toxic oral doses of DDT than are rats fed normal amounts of dietary casein (27%). When the amount of casein in the diet is increased to 81%, the LD50 also declines. The clinical signs of toxicity to DDT at the range of the acute oral LD50 were similar in animals of all dietary groups studied.