A serum factor that promotes migration of (mouse) 3T3 fibroblast cells is shown to be distinct from the growth-promoting and cell-survival factors of serum. The factor promotes migration of cells from the edge of a wound in confluent 3T3 cells, but cell migration under these conditions does not lead to the initiation of DNA synthesis. Subsequent addition of serum initiates DNA synthesis in the migrating cells. The results establish that breaking contacts between quiescent 3T3 cells is not sufficient to initiate DNA synthesis. The DNA synthesis observed in migrating 3T3 cells in the typical "woundhealing" experiment presumably results because the migrating cells have an increased ability to use serum factors. Serum-factor requirements for "wound healing" in cultured 3T3 cells are discussed.