AbstractMxA is a cellular protein activated in cells treated with type I interferons. MxA protein is a member of the dynamin superfamily of large guanosine triphosphatases. A unique property of Mx1 GTPases is their antiviral activity against a wide range of RNA viruses. Replication of several hantavirus strains is inhibited in cells constitutively expressing MxA protein. We have found that Andes virus (ANDV) infection up regulates transcription of MxA RNA and expression of MxA protein in human endothelial cells in vitro. Activation of MxA gene expression requires virus replication. Up‐regulation of MxA gene expression in hantavirus infected cells varies depending on the cell type. The degree of activation of MxA gene expression is inversely correlated with the efficiency of hantavirus replication. Additionally, MxA protein is co‐localized with hantavirus nucleocapsid protein in infected cell. Our data suggest that MxA by interacting with the virus nucleocapsid protein inhibits production of new infectious virus particles. J. Med. Virol. 75:267–275, 2005. © 2004 Wiley‐Liss, Inc.