Ureases (urea aminohydrolase, EC 3.5.1.5) are highly homologous enzymes found in plants, bacteria and fungi. Urease of the jack bean, Canavalia ensiformis, was the first protein ever crystallized and also the first enzyme shown to contain nickel. Canatoxin, an isoform C. ensiformis urease, displays several other biological properties besides its ureolytic activity: 1) insecticidal effects; 2) pro-aggregating activity in rabbit platelets; 3) binding to sialic acid-containing glycoconjugates; 4) pro-inflammatory activity. These effects are independent of hydrolysis of urea and require activation of the eicosanoid metabolism and calcium channels. Microbial ureases play a role in pathogenesis of urinary stones, pyelonephritis, urinary catheter incrustation, peptic ulceration and possibly in gastric tumors. Helicobacter pylori, a Gram-negative bacterium that colonizes the human stomach mucosa, causes gastric ulcers and cancer by a yet not fully understood mechanism. We have investigated if microbial ureases display some of the biological properties described for canatoxin. Ureases from Helicobacter pylori and Bacillus pasteurii induced aggregation of rabbit platelets (ED₅₀ ca. 200 mg/mL and 400 mg/mL respectively), with a time course very similar to the aggregation induced by canatoxin (ED₅₀ ca. 15 mg/mL). Pretreatment of platelets with dexamethasone (phospholipase A₂ inhibitor) and esculetin (12-lipoxygenase inhibitor) blocked aggregation induced by B.pasteurii urease, while indomethacin-treated platelets had an increased response to the microbial enzyme. Thus, similar to canatoxin, platelet aggregation induced by B.pasteurii urease is mediated by lipoxygenase-derived eicosanoids. Preliminar experiments using Helicobacter pylori recombinant urease indicated that its ability to aggregate rabbit platelets is also mediated by lipoxygenase-derived eicosanoids. This finding may help to elucidate the physiopathology of diseases caused by urease-producing bacteria.