Using a new double-labeling immunofluorescence technique, we assessed various growth factors on their ability to promote proliferation of cultured human glial cells. Cells studied were fetal astrocytes, fetal Schwann cells, adult astrocytes, and adult oligodendrocytes. Effective agents for fetal astrocytes were glial growth factor from the bovine pituitary, platelet-derived growth factor, fibroblast growth factor, and 4 beta-phorbol 12,13-dibutyrate. For fetal Schwann cells, mitogens were glial growth factor from the bovine pituitary, platelet-derived growth factor, nerve growth factor, and 4 beta-phorbol 12,13-dibutyrate. Adult astrocytes and oligodendrocytes did not normally divide in culture, and none of the agents tested were effective in inducing their proliferation. The report that interleukin-2 was a mitogen for oligodendrocytes could not be replicated in the present study on any of the glial cell types.