Scrub typhus, caused by infection with Orientia tsutsugamushi, accounts for up to 23% of all fevers in endemic areas of the Asia-Pacific region. Patients often become rickettsemic approximately 1 to 3 days before symptoms of the disease are evident. Infection is common in rural areas and the clinical syndrome can vary from asymptomatic to a fatal illness. In an effort to determine if 0. tsutsugamushi could survive in units of stored blood and present a potential threat to the blood supply, we infected human mononuclear cells isolated from whole blood by density gradient centrifugation and subsequently inoculated them with 0. tsutsugamushi, Karp strain. Infection of the mononuclear cells was determined by Oiemsa stain, direct fluorescent antibody (DFA) staining, and polymerase chain reaction (PCR).