Finally working with rat cell lines, which have been passaged via rats, is also a concern because they can be contaminated by different rat viruses, in general, and by Hantaan virus, in particular. Hantaan virus has been isolated in cell culture from rat immunocytomas. Transplantation into LOU/M/Wsl rats and storage of passaged immunocytomas at -70 • C over a period of 8-10 yr did not eliminate the virus. Lloyd and Jones (1986) also showed that the passage of rat immunocytomas in infected LOU/Wsl rats led to a contamination of these cell lines.