Five hundred clones were picked up from the colonies exposed to pH 4.0, then subjected to urease assay. The urease activity of H. pylori CPY3401 and NHP1 was 353.5 mmol/min/mg and 15.4 mmol/min/mg, respectively. None of the pH 4 plates either from H. pylori and ATCC33394 or H. pylori and KDY1 showed more than 100 mmol/min/mg of urease activity. On the other hand, number of colonies which showed more than 100 mmol/min/mg of urease activity in the pH 4 plate of the H. pylori and NHP1 was 116 (23.2%), indicating CFU/ml of survived H. pylori was 106.15.