Taxol is the prototype of a new class of microtubule stabilizing agents with promising anticancer activity. Several studies show that taxol mimics the actions of lipopolysaccharide (LPS) on murine macrophages. To investigate the mechanism of taxol-induced macrophage stimulation, we evaluated the ability of Rhodobacter sphaeroides diphosphoryl lipid A (RsDPLA) and SDZ 880.431 to block taxol-induced effects. RsDPLA and SDZ 880.431 are lipid A analogues that lack LPS-like activity, but inhibit the actions of LPS, presumably by blocking critical cellular binding sites. We report that RsDPLA and SDZ 880.431 potently inhibited taxol-induced TNF secretion, gene activation, and protein-tyrosine phosphorylation. The role of microtubules in taxol signaling was investigated. Taxol-induced microtubule bundling in primary and transformed RAW 264.7 macrophages was not blocked by RsDPLA or SDZ 880.431. Taxotere, a semisynthetic taxoid, was more potent than taxol as an inducer of microtubule bundling, but did not induce tumor necrosis factor alpha secretion and gene activation. These data dissociate the microtubule effects of taxol from macrophage stimulation and suggest that taxol stimulates macrophages through an LPS receptor-dependent mechanism. The results underscore the potential of taxol as a tool for studying LPS receptor activation and provide insights into possible therapeutic actions of this new class of drugs.