The proliferative response of adult human astrocytes to tumor necrosis factor-alpha (TNF-alpha) was examined. Applied alone, TNF-alpha was dependent on the content of serum in the feeding medium, being mitogenic only in conditions of over 15% serum in medium. In accordance with previous results, supernatants from activated human CD8+ T-lymphocytes (CD8 SN) and recombinant human interferon-gamma (IFN-gamma) enhanced proliferation of adult human astrocytes in 5% serum-containing medium. Simultaneous administration of TNF-alpha (10-1000 units), however, abrogated the mitogenic effects of either CD8 SN or IFN-gamma; the inhibitory effect of TNF-alpha was lost if applied 2 days following IFN-gamma treatment. These studies show that while TNF-alpha is an astrocyte mitogen under selected conditions, it inhibits proliferation induced by other mitogens. In this manner, TNF-alpha may be important in regulating the proliferative response of astrocytes during reactive astrogliosis in vivo.