Using a polyclonal antibody raised against a highly conserved sequence of 38 amino acids containing the activation site (VTDSAAGAT) common to mammalian and yeast alkaline phosphatases (AP), we identified in decapsidated Saccharomyces boulardii a protein phosphatase detected by autoradiography as a single signal (63 kD). Using an affinity chromatography column, the protein phosphatase could be concentrated 39.1-fold and presented as a doublet of two subunits. Compared with rat and bovine purified intestinal AP, the enzyme from S. boulardii had a greater ability to dephosphorylate the lipopolysaccharide (LPS) of Escherichia coli 055B5. When tested in vivo, intraperitoneal injection of intact LPS to rats produced, after 9 h, 100 ng/mL of circulating tumor necrosis factor-alpha with inflammatory lesions and apoptotic bodies in the liver and the heart, whereas rats injected with partially dephosphorylated LPS produced only 40 ng/mL tumor necrosis factor-alpha without organic lesions. In conclusion, S. boulardii is able to inhibit toxicity of E. coli surface endotoxins by the release of a protein phosphatase exhibiting a great capacity of dephosphorylation.