Orientia tsutsugamushi, a causative agent of scrub typhus, is an obligate intracellular parasite. The mechanisms by which O. tsutsugamushi invade host cells are unknown. Given the importance of surface-exposed proteins in the pathogenesis of microbial pathogens, outer membrane proteins (OMP) of O. tsutsugamushi were extracted with detergents and their cellular binding was studied. Outer membrane fraction of O. tsutsugamushi was enriched by a sodium-lauryl sarcosinate (Sarkosyl) treatment of total membranes. Outer membrane proteins were extracted by the treatment with sodium dodecyl sulfate (SDS) and Sarkosyl. The resulting soluble fractions were examined for their cellular binding by the immunofluorescence microscopy. A fifty six kilodalton protein was found to bind to fixed ECV304 cells only when the outer membrane preparation was not treated by DTT or heat. These results suggest that the conformation the 56-kDa OMP is important for the attachment to the host cell surfaces.