The ability of Escherichia coli-derived lipopolysaccharide (LPS), recombinant (r) interleukin 1-beta (rIL-1 beta), and r murine tumor necrosis factor-alpha (rMuTNF-alpha) to induce interleukin 6 (IL-6) production in vivo was investigated. Peak serum IL-6 concentration was attained after 2 hr of LPS injection into mice. The coinjection of antiserum against rMuTNF-alpha with LPS resulted in a reduction of the induced serum IL-6 level, indicating the involvement of endogenous TNF-alpha in LPS induction of IL-6. Recombinant IL-1 beta and rMuTNF-alpha injected directly caused the production of substantial amounts of IL-6 within 30 min. The injection of a combination of rIL-1 beta and rTNF-alpha induced a significantly greater level of IL-6 than either agent alone. The greater level of serum IL-6 was associated with hypothermia and an increased lethality among mice injected with both cytokines. These data demonstrate the abilities of IL-1 beta and TNF-alpha to induce IL-6 production in vivo and indicate that LPS induction of IL-6 may be mediated, at least partially, through TNF-alpha action. The data describe a new in vivo biologic activity shared between IL-1 beta and TNF-alpha and suggest that IL-6 may be an important effector in the manifestation of TNF-alpha and IL-1 beta actions in vivo.