Abstract : Hantaan virus, the causative agent of Korean hemorrhagic fever, is but one of at least four distinct viruses that comprise the proposed genus Hantavirus of the family Bunyaviridae. Three of these viruses are known to cause disease in humans, collectively termed hemorrhagic fever with renal syndrome: Hantaan, Puumala, and urban rat-associated viruses. Hantaviruses cause asymptomatic, chronic infection in their rodent hosts and presistent, noncytolytic infection in cells grown in culture. Transmission among rodents and to humans is thought to occur via aerosolized infectious urine, saliva, or feces or by bites. Once infected, rodents appear to be infected, and probably are infectous, for life. Outbreaks of disease in humans associated with infected colonies of laboratory rats have been reported. A hantavirus has been isolated from a tumor cell line passaged in infected laboratory rats, and transmission of hantaviruses during passage of rat tumor cell lines was confirmed. Clearly, the existence of hantaviruses as adventitious agents in cell cultures of rat origin or antibody-secreting cells could pose a health hazard to humans as well as potential confounding variable during experimentation. Realizing this, we set out to determine if hantaviruses were present in certified cell lines of rat origin housed at the ATCC (Rockville, Md), the official American repository for such cell lines.