In intraspecific incompatibility systems of the sporophytic type and in certain forms of interspecific incompatibility, the inhibition of the pollen or pollen tubes takes place on the stigma surface. The 'recognition' event involves proteins carried in the pollen wall, apparently in interaction with a proteinaceous receptor layer on the outer face of the walls of the stigma papillae. Stigma surface proteins have been detected by various techniques in species of more than 80 angiosperm families, and it seems that they are a characteristic feature of the group. The pellicle proteins, which can be stripped with SDS, have been partly characterized. An account is given of the nature and origin of the pellicle, and its likely function in mediating incompatibility responses discussed.