The effect of cytokines on astrocytes cultured from mature bovine brain was determined both in a serum-containing medium and in a chemically-defined medium. The results showed that in serum-free medium, human TNF and, to a lesser degree, IL-6 and lymphotoxin, were mitogenic for astrocytes. Increased uptake of [3H]thymidine could be detected within 36 h in vitro and its presence in astrocytes was confirmed by autoradiography. In contrast, neither IL-1 alpha nor IL-1 beta induced astrocyte proliferation in serum-free medium but showed some synergistic effect with serum after 72 h. The proliferative effect of TNF and IL-6 was confirmed by cell counting. None of the cytokines tested was toxic for astrocytes as measured by 51Cr release. No mitogenic effect for oligodendroglia, purified from the same source, was detected. The results support a role for products of activated inflammatory cells in the development of astrocyte proliferation that may contribute to the reactive gliosis found in white matter diseases of the central nervous system such as multiple sclerosis.