Studies were conducted to define the natural host range of the Korean hemorrhagic fever (KHF) agent in South Korea, and to identify colonized rodents susceptible to this infection. Eight species of field rodents were captured in areas of Korea endemic for KHF and their tissues were examined by immunofluorescence for the presence of KHF antigen. One hundred and fourteen of 817 Apodemus agrarius coreae captured between 1974 and 1978 had one or more positive organs. No positive organ was found in 239 rodents of the other seven species examined. Two hundred and thirty-eight specimens of Apodemus agrarius jejuensis captured on Jeju Island, an area thought to be free of disease, were also negative. Attempted laboratory infection of nine species of rodents captured in the field but maintained in the laboratory was successful only in the two subspecies of Apodemus. The 46 specimens of A. a. jejuensis tested in this manner were all uniformly susceptible to infection as determined by immunofluorescence. Serial sacrifice of experimentally infected A. a. jejuensis revealed viremia of short duration terminating on day 10 postinfection. In contrast, other tissues of this animal, including lung, kidney, liver and parotid gland were positive on day 10 and remained so through the 100-day observation period. When 12 species of colonized laboratory rodents were inoculated with KHF agent five were found to develop KHF antibody by indirect immunofluorescence and two, Calomys callosus and Apodemus agrarius ningpoensis, developed detectable KHF antigen in their tissues.