Many higher plants have developed systems to prevent self-fertilization. The self-incompatibility (SI) system has been well known since the time of Darwin. In Brassica, SI is controlled by a single polymorphic locus called S. Discrimination of self and non-self pollen by the pistil should occur on the surface of the stigma. We have investigated the molecular mechanism underlying the SI system and have determined the S-gene products in pistil and pollen. The S-determinant of pollen (SP11) is a small cysteine-rich protein and is the ligand of a receptor complex in papilla cells, which is composed of S-receptor kinase (SRK) and S-locus glycoprotein (SLG). Ligand binding induces the autophosphorylation of SRK, which triggers the signaling cascade that results in the rejection of self-pollen. The ligand-receptor interaction occurs in an S-haplotype specific manner. This is the fundamental reaction of self-recognition system underlying SI in Brassica.