In order to produce an ELISA kit for detection of antibodies against O tsutsugamushi in Vietnam, four truncated 56 kDa antigenic genes of O tsutsugamushi including Karp (HT-09), Gilliam (HT-11), TA763 (HT-49), and Kato (YB-50) íolated from the most prevalent cases in Vietnam were cloned and expressed in E coli Rosetta 1 cells.