An evaluation of human recombinant interferon beta (rIFN-β) against selected positive- and negative-stranded RNA viruses using an MTT assay procedure : J.J. Kirsi 1, W.M. Shannon 1, T.P. Monath 2, and J. W. Huggins 2. Southern Research Institute, Birmingham, AL 35255 USA 1, and U.S. Army Medical Research Institute of Infectious Diseases, Fort Detrick, Frederick, MD 21701 USA 2