Abstract 1. 1. It has previously been shown that the growth rate in fresh medium of secondary cultures of chick embryo fibroblasts is positively correlated with their cell concentration, over a wide range; and that sparse cultures grow much more rapidly in medium conditioned by incubation with concentrated cell cultures than in fresh medium. 2. 2. We now report that the growth of a small number of cells is greatly enhanced by plating the cells in a small area of the culture surface. Indeed, a relatively small number of cells in fresh medium will grow at their maximum rate if plated at a high enough concentration per unit area. Cultures made with equal concentrations of cells per unit area, but with 25-fold differences in total cell number, showed approximately the same relative increase in cell number in the first day after plating in fresh medium. Thus the growth of a culture in this period is determined largely, and possibly entirely, by its concentration of cells per unit area. 3. 3. When coverslips with sparse cultures of living cells are inverted and supported above moderately hevy feeder layers, it is found that the extent of growth enhancement transmitted from the feeder cells to the living cells decreases with increasing distance to the coverslip. 4. 4. It is suggested that this short-range growth enhancement occurs because the conditioning factor is at a significantly higher concentration in the medium near the cells than in the bulk of the medium, and that this concentration difference is a consequence of the low rate at which the conditioning factor diffuses away from the cells. 5. 5. Possible analogies between the short-range growth enhancement and other types of interaction between vertebrate cells are discussed.