Fil: Lewis, Adrian P. ANLIS Dr.C.G.Malbrán. Instituto Nacional de Enfermedades Infecciosas. Departamento de Virología; Argentina.Fil: Bok, Karin. ANLIS Dr.C.G.Malbrán. Instituto Nacional de Enfermedades Infecciosas. Departamento de Virología; Argentina.Fil: Perez, O. ANLIS Dr.C.G.Malbrán. Instituto Nacional de Producción de Biológicos; Argentina.Fil: DeFillippo, J. ANLIS Dr.C.G.Malbrán. Instituto Nacional de Producción de Biológicos; Argentina.Fil: Paolazzi, C. ANLIS Dr.C.G.Malbrán. Instituto Nacional de Producción de Biológicos; Argentina.Fil: Gomez, Jorge A. ANLIS Dr.C.G.Malbrán. Instituto Nacional de Enfermedades Infecciosas. Departamento de Virología; Argentina.Due to recent political developments, smallpox has re-emerged as a serious threat. We recovered and characterized an old batch of smallpox vaccine, Malbrán strain, produced between 1945 and 1949. The virus was re-isolated and characterized by sequence analysis and biological activity in animals. Phylogenetic analysis using the hemagglutinin and A45R genes showed that the Malbrán strain was closely related to the Lister strain of vaccinia virus. In animals, the Malbrán strain exhibited low pathogenicity, confirming historical records. Mice immunized with the Malbrán strain survived a lethal challenge with cowpox virus. Thus, this strain of vaccinia virus remains a viable candidate as a smallpox vaccine.