When small explants from early gastrulae of Xenopus laevis are allowed to develop in a buffered salt solution there is a considerable difference between the patterns of differentiation obtained from different dorsoventral levels of the marginal zone. These patterns of differentiation correspond to the fates of the different regions in the course of normal development. They are not altered if several explants of the same type are fused before culture. If a ventral marginal zone explant from Xenopus is cultured in contact with a piece of dorsal marginal zone from the axolotl, it forms structures more dorsal in character than it would in isolation or in normal development. This behaviour is shown only feebly with other regions of the axolotl gastrula. A piece of dorsal marginal zone from Xenopus is not affected in its development by culture in contact with an explant of ventral marginal zone from the axolotl. The dorsalization of ventral marginal zone explants is shown only if there is a large area of direct contact with the dorsal explant and if the pieces remain in contact for a period of 48 h or more. It is proposed that this in vitro interaction is the same as the dorsoventral component of action of the 'organizer' graft discovered by Spermann and Mangold.