We identified the cytokines interleukin-1 beta (IL-1 beta), tumor necrosis factor (TNF), and interleukin-6 (IL-6) by specific radioimmunoassays in the CSF of patients with multiple sclerosis (MS) and other neurologic diseases (OND). There was a high incidence of detectable IL-1 beta in patients with active MS compared with inactive MS or OND patients. TNF was also more frequently present in active MS than in OND CSF. By contrast, most MS CSF did not contain detectable IL-6. There was no correlation between the degree of CSF pleocytosis and the level of individual cytokines, suggesting that cytokine accumulations may be derived from CNS, and not CSF, cells. As IL-1 beta and TNF experimentally induce astrogliosis, demyelination, temperature elevation, lassitude, and sleep, and results raise the possibility that these cytokines may contribute to a variety of manifestations in MS and in other disease states.