Con A induced dorsal differentiation in the ventral mesoderm of Cynops gastrula embryo. This process apparently requires a certain amount of Con A to be internalized as supported by the following evidence: 1) Oligomannose‐type oligosaccharide, a potent inhibitor of Con A, considerably inhibited dorsalization of ventral mesoderm by Con A. The incorporation of 125I‐Con A into the ventral mesoderm was greatly inhibited by this sugar. 2) Sepharose‐immobilized Con A did not dorsalize the ventral mesoderm. Con A‐induced dorsalization was found to be concentration‐dependent. Microautoradiograms of 125I‐Con A‐treated ventral mesoderm suggest that the target site (some receptor molecules) of Con A exists inside the cell. Con A is the first pure substance reported to mimic the two properties of the organizer—neural induction of the competent ectoderm and dorsalization of the ventral mesoderm. In neural induction, Con A acts on the cell surface, while Con A apparently needs to be internalized to trigger dorsal differentiation. Interestingly, Con A‐dorsalized ventral mesoderm acquired the neural inducing function of the organizer within the early phase of dorsalization.