Experiments were designed to discriminate between inhibition of growth due to contacts or exhaustion of serum factors. The cell layer was wounded and the migrating cells were followed by time-lapse cinematography; DNA synthesis in the same cells was recognized by means of (3)H-thymidine labeling and radioautography. In this way, the complete history of individual cells migrating to the wound could be described. The results show that topographical relationships between cells play an important role in controlling initiation of DNA synthesis. It is still unclear whether initiation is promoted by release from contacts or by the increased ability of the cells to utilize serum factors because of their changes in shapes and activities.