The genome of a strongly attenuated vaccinia virus strain, MVA, was investigated by Southern blot and sequence analyses. Three major deletions, relative to the WR strain, were localized in MVA DNA. The deletions occurred near both ends of the viral genome and one of them affected a 55 K as well as the 32 K human host range gene. Although more than two thirds of the host range gene were eliminated from the MVA, the virus could still multiply in certain human cells.