To determine the existence of an acid stress response in Helicobacter pylori the global changes in the proteins synthesized by the bacterium when subjected to an acid stress were studied. H. pylori ATCC43504 previously adapted to pH 7 did not show an acid stress response as detected by the two-dimensional electrophoretic pattern of 35S-labeled proteins when incubated at pH 3. This was probably due to the neutralization of the external medium by the action of urease. However, H. pylori DW504UreI-negative, a mutant strain unable to transport urea into the cell, showed a large number of proteins changed, as is typical in an acid stress response. Some of these proteins were identified by N-terminal sequencing.