The PCR primer set JW21-JW22 of Weiss et al. (19), which was reported to amplify a 139-bp fragment of the l6S rRNA gene of Helicobacter pylori, has been recently used for the detection of H. pylori in clinical specimens. However, when we applied JW21-JW22 PCR to other members of the genus Helicobacter and unrelated microorganisms, all of these bacteria produced a 139-bp PCR product. Therefore, we designed a novel primer set, HPU185-HPL826, which produced a 642-bp amplicon of the l6S rRNA gene of H. pylori. Then we further examined the specificity of the novel PCR assay using Southern blot hybridization with an internal probe, HPP225. The PCR assay described in this study was shown to be highly sensitive and specific only to the H. pylori 16S rRNA gene sequences.