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Summary Chicks were fed complete diets containing protein exclusively from soybean meal, either cooked or raw or in combinations, at a total of 20% protein. As little as 5% of raw protein was accompanied by nearly maximal growth retardation due to the tryptic inhibitor in raw soybean meal. The growth inhibition was overcome by adding trypsin to the diet.

The well-known improvement in nutritive value of raw soybean protein on heating or methionine supplementation has been reproduced in chick diets containing the soybean antitrypsin, but deficient in lysine, arginine, or isoleucine, by heating to destroy the antitrypsin or by adding the particular amino acid inadequately supplied in the diet. The mechanism of this effect has been shown to be a general one and probably applicable to any indispensable amino acid.