The major trypsin inhibitor present in soybean seed [Glycine max (L.) Merrill] is the Kunitz trypsin inhibitor or soybean trypsin inhibitor A2 (SBTI-A2). Four types of SBTI-A29 have been identified in the U.S. soybean germplasm collection. Three of the types designated Tia, Tib, and Tic are electrophoretically distinguishable from one another by their different Rf values of 0.79, 0.75, and 0.83, respectively. The three types are inherited as codominant alleles in a multiple allelic system at a single locus. The fourth type which is the absence of SBTI-A2 is found in P.I. 157440 and P.I. 196168. The gene for the lack of SBTI-A2 is designated ti and is inherited as a recessive allele to the other three SBTI-A2 9 types. Tia is the most common SBTI-A2 type in the germplasm collection. Linkage studies revealed that the SBTI-A2 protein is inherited independently of certain morphological characters and chemical components of seed. Potential applications of the SBTI-A2 types are discussed.