Lysates of X61, a striatal-derived cell line, and a partially purified preparation from the lysate (UF4) contain a factor(s) capable of increasing the dopamine content of a mesencephalic-derived dopaminergic cell line (MN9D) and of cultures containing primary dopaminergic neurons. Treatment of cultures containing dopaminergic primary neurons grown in the absence of target cells over a 2 week period with X61 lysate or UF4 resulted in an elevation of dopamine levels of the cultures and of media homovanillic acid as well as a 2.0-fold (UF4) to 2.9-fold (X61 lysate) increase in the density of dopaminergic neurons in treated cultures. The results suggest that the activity factor derived from X61 is capable of preventing dopaminergic cell loss which occurs in the absence of dopaminergic target cells of the corpus striatum.