Lipopolysaccharide (LPS) from Escherichia coli was found to synergize with human recombinant tumour necrosis factor-alpha (TNF-alpha) in the lysis of L929 and WEHI 164 (clone 13) murine fibroblasts, two cell lines classically used in TNF-alpha bioassays. The effect was noted with TNF-alpha at low (sublytic or lightly lytic) concentrations and was significant for LPS concentrations in the ng range. The LPS effect could be inhibited by polymyxin B, and was not observed when the TNF-alpha assay was performed in the absence of actinomycin D. Enhancement of TNF-alpha lysis by LPS occurred in several assays for determining TNF-alpha, including MTT cleavage, crystal violet staining and lactate dehydrogenase release. Synergism was obtained only when LPS and TNF-alpha were added to cells simultaneously, but not when applied in sequence. The reported synergism may be relevant for TNF-alpha determinations by bioassay, and for the understanding of pathophysiology of Gram-negative sepsis.