The stigmas of buds of the age range capable of accepting self-pollen (bud pollination) in self incompatible individuals of Raphanus sativus and Cheiranthus cheiri were found to have cuticularised papillae already carrying a thin surface secretion over the receptive area. Enzymatic digestion of the proteinaceous part of this secretion reduced the germination rate of both compatible and incompatible pollen and prevented the tubes that were formed from penetrating the cuticle of the papillae, so abolishing the self compatibility. This indicates that in buds of an age capable of accepting self-pollen the stigma surface factors concerned with the self-incompatibility response have not yet accumulated in quantities sufficient to inhibit pollen germination and tube growth, although those involved in promoting germination and facilitating the entry of the tubes are already present. The behaviour of the Cruciferae which do not accept self-pollination in buds of any age, such as a variety of Sinapis alba tested, may be explained on the assumption that the stigma surface factors concerned in germination and tube penetration are secreted simultaneously with the incompatibility factors so that there is no period when the incompatible pollen and pollen tubes are both free from the inhibitions imposed by the self-incompatibility system and capable of penetrating the stigma.