We have obtained a highly purified astrocyte population in cultures originating from neonatal (2-5 days) rat cerebrum by use of the selection process provided by a serum-free chemically defined medium (DM). The addition of a glial growth factor isolated from bovine pituitary glands to DM induced in these astrocyte cultures both a stimulation of astrocytic proliferation and a morphological transformation of the astrocytes from flat fibroblastic form to multipolar stellate form.