The effects of transforming growth factor-β (TGF-β) and ACTH on growth, as indicated by [3H]thymidine incorporation into DNA, of primary cultures of neocortical cells from the human fetal adrenal gland were studied. TGF-β inhibited, in a dose- and time-dependent manner, the growth of fetal neocortical cells, and ACTH significantly blunted the inhibitory effects of TGF-β on growth of these cells. ACTH did not block the inhibitory effects of TGF-β on growth of fetal adrenal fibroblasts or liver cells; neither ACTH nor TGF-β had any effect on growth of fetal kidney cells. Thus, it appears that growth regulation of the neocortex may differ strikingly from that of the fetal zone of the human fetal adrenal, in which ACTH and TGF-β have been reported recently to have additive inhibitory effects on cell proliferation. © 1992 by The Endocrine Society.