The ability of various strains of Coxiella burnetii (C.b.) and their phase I and II lipopolysaccharides (LPSs) to induce tumor necrosis factor alpha (TNF-alpha) in peritoneal Balb/c mouse macrophages in vitro was investigated. Considerable differences in the induction ability were observed in dependence on the strain applied. In a TNF-alpha bioassay, the most effective inducers were both corpuscles and LPSs of the strains Priscilla and Scurry, followed by Nine Mile, Luga, and Henzerling I. In contrast, in ELISA, the most effective inducers were LPSs of the strains Luga and Henzerling, followed by Nine Mile, Priscilla, and Scurry. The role of toll-like receptor 4 (TLR4) in the induction was confirmed by the use of C3H/HeJ mouse macrophages. Thus, the induction of TNF-alpha was much higher in Balb/c mouse macrophages than that in TLR4-deficient C3H/HeJ mouse macrophages. Differences in the results of the bioassay and those of ELISA suggest a role of another secreted factor(s) induced with C.b. in murine macrophages that could act synergically with TNF-alpha in L929 cells in the bioassay. The observed differences in TNF-alpha induction might play a role in the pathobiology of Q fever.