Proteins of soybeans (Glycine max) are widely used in animal and human nutrition. In addition to the bulk of the seed storage proteins, which are classified as albumins and globulins, approximately 6% of soybean proteins are classified as inhibitors of trypsin and chymotrypsin and approximately 0.5% are sugar-binding lectins. The two major classes of inhibitors are the Kunitz trypsin inhibitor, which inhibits trypsin, and the Bowman-Birk inhibitor (BBI), which inhibits both trypsin and chymotrypsin. Unless removed or inactivated, these inhibitors and lectins can impair the nutritional quality and safety of soy-based diets. On the other hand, several studies suggest that BBI can also function as an anticarcinogen, possibly through interaction with a cellular serine protease. Good-quality soybean proteins contribute to the nutritional value of many specialty foods including infant soy formulas and milk replacers for calves, and provide texture to many processed foods. However, they may also induce occasional allergic responses in humans. This paper outlines immunoassays developed to analyze for soy proteins in different soybean lines, in processed foods, and in nonsoy foods fortified with soy proteins. An assessment of the current status of immunoassays, especially of enzyme-linked immunosorbent assays for soybean inhibitors of digestive enzymes, soy globulins, and soy lectins, demonstrates the usefulness of these methods in plant and food sciences and in medicine.