Abstract: A cytotoxin associated immunodominant antigen and the nucleic acid encoding the antigen from Helicobacter pylori are described. This antigen was identified from the cytotoxin positive CCUG 17874 Helicobacter pylori strain, and both the antigen and the DNA encoding it have been sequenced. The antigen is a hydrophilic, surface-exposed protein having a molecular weight of 120-132 kDa. The nucleic acid encoding the antigen may be incorporated into a vector for transformation of host cells for expression of the antigen. Both the DNA and the antigen can be used in assays for detection of disease or infection by Helicobacter pylori, and may find use in treating and preventing infection by Helicobacter pylori and the diseases associated with such infection.