The present invention relates in part to antibodies for the performance of immunoassays to determine whether a test sample, such as a food sample, contains tissue derived from a Pangasius species of fish, such as tra or basa. Such antibodies, either alone or in combination, specifically bind to a thermostable antigen from a Pangasius species. The present invention further relates to methods for conducting immunoassays that may use such antibodies to detect the presence of a test antigen in a test sample that is derived from a Pangasius species of fish, such as tra or basa. In addition, such antibodies may be part of a test kit that may also contain one or more test reagent(s) or items of test equipment.