Food provides a continuous antigenic stimulus to the immune system and the antigenicity of processed food proteins should be considered in toxicological evaluations. The antigenicity of the Kunitz trypsin inhibitor was studied using antibodies prepared by inoculating rabbits with native, heat-denatured, and N-acetylcysteine-treated Kunitz soybean trypsin inhibitors. Immunochemical studies using a competitive solid-phase enzyme immunoassay and two groups of sera revealed two patterns of antigenicity. Antibodies elicited with the denatured inhibitor were specific for the denatured conformation of the protein. In contrast, native inhibitor elicited antibodies that selectively recognized determinants in both native and heat-treated protein, but that did not bind trypsin inhibitors treated with N-acetylcysteine. These results imply that: the disulfide bonds must be intact to maintain the native antigenic conformation and the cysteine treatment may suppress allergic manifestations of soybean trypsin inhibitors and possibly other food proteins. These studies were extended by analyzing a panel of monoclonal antibodies prepared against native Kunitz trypsin inhibitor. The inhibitor has at least two distinct antigenic sites (epitopes), one of which is retained under denaturing conditions. The measurement of native Kunitz trypsin inhibitor in food samples by immunoassay appears practical. The relevance of these findings to food processing, food safety, and health is also discussed.