Modified virus Ankara (MVA) is a vaccinia virus (VV)strain that was attenuated by serial passage through chick embryo fibroblasts (CEFs) and contains six large genomic deletions compared with parental virus. MVA replicates well in CEFs, but poorly in most mammalian cells. Recombinant MVA is a promising human vaccine candidate due to its restricted host range, immunogenicity and avirulence in animal models, and excellent safety record as a smallpox vaccine. Here we present a further characterization of MVA and demonstrate that : (i) MVA can replicate, albeit poorly, in transformed human cell lines, but not in primary human fibro-