Publisher Summary Lima bean (Phaseolus lunatus) trypsin inhibitors comprise at least four biologically active variants. This chapter includes information on the amino acid sequence of the inhibitors, on their reactive sites, and on their interactions and complexes with trypsin and chymotrypsin. It discusses the kinetic properties, the physical properties and the amino acid sequence. The methodology of monomer has an apparent molecular weight of about 9,000, but it undergoes a concentration-dependent dimerization. Reactive sites and chemical modifications are discussed in the chapter, and it comes up with the conclusion: the modified inhibitor obtained after dissociation of reduced and carboxymethylated trypsin-inhibitor complex is fully active against both trypsin and chymotrypsin.