After their ingression, the primary mesenchyme cells (PMCs) of the sea urchin embryo migrate within the blastocoel, where they eventually become arranged in a characteristic ring-like pattern. To gain information about how the movements of the PMCs are regulated, a microinjection procedure was developed and used to transplant PMCs to novel environments within embryos of different developmental stages. Donor PMCs were vitally stained with rhodamine isothiocyanate so their movements could be followed. When PMCs were transplanted into the blastocoel of embryos at the mesenchyme blastula stage, the donor cells moved to the vegetal region and joined with the host's PMCs in forming a normal ring pattern and skeleton. When PMCs were microinjected into the blastocoel of early (prehatching) blastulae, they moved toward and accumulated in the vegetal region of the host embryo within 3 hr. They did not, however, elaborate a complete ring pattern when they would have done so if left in situ. Instead, they participated in the formation of the ring pattern that was subsequently formed by the PMCs of the host embryo. These data indicate that information present in the host embryo and established well before the mesenchyme blastula stage guides the donor PMCs to the vegetal region of the blastocoel. Subsequent changes in the host environment, however, are necessary for the formation of the ring pattern. When PMCs were transplanted into older hosts (late gastrula or prism stage embryos), they exhibited a reduced capacity for elaborating a correct pattern. Their ability to do so was dependent upon their initial placement within the blastocoel, and may reflect a restricted ability on the part of the PMCs to migrate within the blastocoel of a developmentally "old" embryo. The specificity of the interaction between the PMCs and their migratory environment was examined by microinjecting latex beads, ectoderm cells, and secondary mesenchyme cells into the blastocoel of prehatching and mesenchyme blastulae. Neither the latex beads nor the transplanted cells migrated like the PMCs, indicating that the interaction between the PMCs and their migratory environment is a highly specific one.