Sonically disrupted chick embryo cells markedly stimulate DNA synthesis and cell multiplication when added to the medium of population density-inhibited cultures of such cells, but have little effect on the growth rate of sparse cultures. Sonicates from density-inhibited chick embryo cultures have as much overgrowth-stimulating activity as do sonicates from actively growing cells. Sonicates from cells infected with Rous sarcoma virus show markedly increased overgrowth-stimulating activity 4 days after infection. The activity in Rous-cell sonicates falls back to near normal amounts at 6 days concurrently with the appearance of a high content of overgrowth-stimulating activity in the medium. The active material is nondialyzable. It seems that growth-inhibited cells contain material in a sequestered location which, when released, can stimulate rapid growth in similarly inhibited cells.