The 56-kDa major outer membrane protein antigen of Orientia tsutsugamuchi is the immunodominant antigen in human scrub typhus (ST) infections. An enzyme-linked immunosorbent assay (ELISA) using a recombinant 56-kDa protein (r56) to detect specific immunoglobulin M (IgM) produced in ST infections was developed, and its performance was evaluated using sera from patients with active ST (n = 59), spotted fever (SF) (n = 31), and murine typhus (MT) (n = 6) and from those without rickettsial infection (n = 52). The r56 ELISA was compared to an ELISA using native whole cell lysate of O. tsutsugamushi Karp or O. tsutsugamushi Gilliam as antigens. The performance of the assays using r56 was similar to that of those using native antigens. Using indirect immunoperoxidase (IIP) as the reference test, sensitivities were 86, 88, and 88% while specificities were 84, 90, and 87% in the three assays. Furthermore, cross-reactivity in confirmed cases of SF and MT was low (5.4, 2.7, and 2.7% respectively). The additional use of IgG in the r56 ELISA gave improved performance (sensitivity, 80%; specificity, 96%; cross-reactivity in SF and MT, 2.7%). The detection of high levels of IgG in some IgM-negative patients illustrates the importance of including a test for IgG in the detection of secondary or reactivated infections, since many of these patients were from regions in Thailand where these infections are endemic.