A specific intercellular interaction has been demonstrated between neuronal and non-neuronal cells that appears to increase the rate of non-neuronal cell proliferation. Isolated and recombined primary cultures of both cell types were prepared from 11-day embryonic chick sympathetic ganglia by a method recently developed in this laboratory. When non-dividing neurons were added to an equal number of proliferating non-neuronal cells, the amount of [methyl-3H]thymidine incorporated by these mixed cultures was 230% greater than that incorporated by 99% pure non-neuronal cultures. Removal of all neurons from such non-neuronal cultures by a 48-h preincubation without nerve growth factor resulted in an even greater increase in [3H]thymidine incorporation upon addition of neurons (370%). When increasing numbers of isolated neurons were added to non-neuronal cell cultures, the amount of [3H]thymidine incorporation initially increased in a dose-dependent fashion until it reached a plateau. In contrast, the addition of increasing numbers of non-neuronal cells to a constant number of neurons resulted in a linear increase in [3H]thymidine incorporation. In some cases neurons and non-neuronal cells were not grown in direct physical contact but were only allowed to communicate with one another through the culture medium. Such indirect communication never resulted in a stimulation of [3H]thymidine incorporation. When neurons were added to cultures of embryonic chick fibroblasts, the neurons grew well but did not stimulate [3H]thymidine incorporation by the fibroblasts. These results suggest that embryonic sympathetic neurons selectively stimulate the proliferation of non-neuronal cells derived from the same source.