The urease of Helicobacter pylori is suspected to play a role in the pathogenesis of gastritis. Although all clinical isolates of H. pylori are urease positive (U+), we have selected and characterized several spontaneously arising urease-negative (U-) variants from wild-type strain 60190. Urease-negative variants were identified by growth in medium containing 60 mM urea and arose at a frequency of 10(-5) to 10(-6). The urease activity of the wild-type strain inhibited growth of this strain in the presence of 60 mM urea. U- variants retained the U- phenotype for more than 100 passages on medium with or without urea. The urease activities of the original U+ and derived U- cells were 9.55 to 16.7 and 0.01 to 0.17 U/mg of protein, respectively. Colonial growth and other biochemical characteristics were identical for the strains. U- variants showed three classes of whole-cell sodium dodecyl sulfate-polyacrylamide gel electrophoresis profiles: (i) identical to U+; (ii) change in the migration of the 61-kDa urease subunit; and (iii) lack of 61- and 30-kDa subunits. These differences were confirmed by immunoblotting and by protein separation using fast protein liquid chromatography. The U+ strain but not U- variants tolerated exposure to pH 4.0 for 60 min in the presence of urea. Supernatants of the U+ strain and U- variants contained vacuolating cytotoxin activity for HeLa cells in similar titers. By enzyme-linked immunosorbent assay, human serum samples recognized water extract from the U+ strain significantly better than extract from a U- variant lacking urease subunits. In conclusion, this study demonstrates that U- H. pylori variants may arise spontaneously, that urease activity enhances survival at acid pH, and that urease and cytotoxin activities are disparate phenotypes.