For soybeans to serve as a good source of protein for feeding animals as well as humans, a certain amount of heat treatment or some other form of processing must be applied. This is because there are present in soybeans certain heat-labile factors that exert an adverse effect on the nutritional value of the protein. The so-called protease inhibitors have received the most attention in this regard and have been shown to exert their antinutritional effect in the short term by causing pancreatic hypertrophy and hyperplasia in the rat, the underlying cause for an inhibition of growth in these animals. The prolonged feeding of raw soy flour or an enriched trypsin inhibitor fraction from soybeans to rats results in the development of hyperplastic and neoplastic nodules of the pancreas, including carcinomas. It should be emphasized that all of these adverse effects are seen when protease inhibitors are present in relatively high concentrations in the diet and may be completely unrelated to the anticarcinogenic effects seen at low concentrations of the Bowman-Birk inhibitor. Brief mention is also made of any possible adverse effects that may result from the presence of phytic acid and saponins in soybeans.