The pollen grain coating of Brassica oleracea contains a polymorphic family of highly charged small proteins (PCP-A, pollen coat protein, class A) related to the defensin class of seed proteins. On pollination these proteins are released from the grain and in vitro data suggest that at least one member of the family (PCP-A1) interacts specifically with elements of the stigmatically-expressed S(self-incompatibility) receptor complex. A new in vivo bioassay has demonstrated the male determinant of the self incompatibility system to be contained within the pollen coating, and this determinant to be a low molecular mass protein. A combination of data from interspecific studies and molecular analysis of PCP-A proteins indicates that the primary interaction between PCP-A1 and the receptor complex may be involved in establishing compatibility, while other molecular interactions, perhaps involving other PCP-A class proteins, are responsible for regulating S-specific rejection of self grains. The evolution of the self incompatibility system on the dry sigma of Brassica is discussed in the context of these data.