The early chick blastoderm has been used as a rapidly proliferating organised system of differentiating cells. From the present study, it is noted that copper is transported into the blastoderm through the periblastodermal yolk globules. Over a large dose range the changes are identical, indicating that the yolk monitors the movement. Mordanting of haematoxylin and histochemical staining indicate that copper is bound to nucleic acids during transport. The changes produced by this trace metal appear to be a reflection of this binding. The possible role of this finding in neoplasia has been discussed.