The specific rate of inactivation of antitoxin in urea solutions, as measured by the Römer neutralization test with toxin, has been shown to be independent of the concentration of protein under the conditions studied. The amount of precipitate obtained in the quantitative precipitation test with toxin, however, increases greatly with increasing protein concentration during denaturation. The time during which the protein concentration is important in this respect has been shown to be the interval in which the urea is being dialyzed from the solutions. The meaning of the results is discussed.

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