In a normal tissue culture medium containing 10% fetal calf serum, purified rat Schwann cells divide very slowly. We have previously reported that the cells are stimulated to divide by an activity present in extracts of the brain and pituitary (Brockes, J.P., Fields, K.L. and Raff, M.C. (1979) Brain Res. 165, 105-118), and this activity appears to be both novel and restricted in its distribution (Raff, M.C., Abney, E.R., Brockes, J.P. and Hornby-Smith, A. (1978) Cell 15, 813-822). The pituitary activity has been purified over 4000-fold from a pool of 10 kg of frozen glands and 4000 lyophilized anterior lobes. The activity was assayed by the incorporation of 125I-UdR into DNA of Schwann cells growing in microwells. The most purified (phosphocellulose) fraction was analyzed on sodium dodecyl sulfate-polyacrylamide gel electrophoresis and displayed a major band of 3 x 10(4) molecular weight which was approximately 25% of the stained material. When analyzed by native gel electrophoresis at pH 4.5 followed by a second dimension of sodium dodecyl sulfate-gel electrophoresis, the activity was consistently associated with this component. The effect of the phosphocellulose fraction on proliferation of central glial cells in dissociated cultures of the rat corpus callosum was also investigated by using fluorescent antisera to identify the cells and [3H]thymidine autoradiography to assay proliferation. The oligodendrocytes and “macrophage-like” microglia were not significantly stimulated, but the astrocytes were stimulated over the same range of concentration as Schwann cells. The activity against astrocytes and that against Schwann cells co-migrated in native gel electrophoresis at pH 4.5, providing strong evidence that the same molecule acts on both cell types.