The abilities of the different cells from human central nervous system (CNS) to produce IL-6, IL-1 beta, and TNF-alpha were tested in vitro using either cultures enriched in human embryonic microglial cells or primary cultures of human embryonic CNS cells. High amounts of IL-6, low amounts of IL-1 beta but no TNF-alpha were detected in supernatants of microglial cells, kept either in FCS-free conditions or in FCS-containing medium. Moreover, IL-6 mRNA was also present in 45 to 55% of microglial cells cultured in the presence of FCS as visualized by in situ hybridization, whereas IL-1 beta mRNA remained undetectable. After prestimulation of microglial cells with LPS or IL-1 alpha, the percentage of cells labeled with an antisense IL-6 mRNA probe increased to 70% and hybridization with an antisense IL-1 beta mRNA probe became detectable. In contrast to this dyscoordinate production of cytokines by microglial cells, human monocytes, freshly isolated from blood and kept in the same culture conditions, produced high levels of the three cytokines tested. In primary cultures of human embryonic CNS cells, IL-6, IL-1 beta, and TNF-alpha were produced mostly or only by microglial cells because no IL-1 beta mRNA or IL-6 mRNA were detected in astrocytes, even after prestimulation with LPS or IL-1 alpha. Finally, IL-1 was the main inducer of IL-6 production because IL-1 alpha, but not LPS, induced a significant increase in IL-6 synthesis in cultures kept in FCS-free medium. However, in presence of FCS, LPS appeared to initiate a cascade reaction involving the production of IL-1 by microglial cells, acting as an autocrine loop to trigger IL-6 synthesis.