Porcine cells treated with interferon (IFN) or double-stranded RNA synthesize two proteins that exhibit high homology of the amino acid sequence to mouse Mx1 protein involved in selective resistance to influenza virus. A full-length cDNA clone (poMx1) encoding the porcine Mx1 protein was isolated and sequenced. It contained an open reading frame of 663 amino acids. The predicted molecular weight of 75.6 kD is in good agreement with the apparent molecular mass of the two immunoprecipitable proteins of 76 kD and 73 kD determined by SDS polyacrylamide gel electrophoresis. A second cDNA (poMx2) was characterized which was incomplete in the 5' region. A comparison of all known Mx proteins revealed an average homology of 67.5%. The porcine Mx1 polypeptide is most closely related to human MxA (p78), murine Mx2, rat Mx2, and rat Mx3 proteins. The amino-terminal halves of all Mx proteins are highly conserved and possess three consensus elements in proper spacing, characteristic of GTP-binding domains. The Mx family shows in their amino termini striking homology to previously characterized Mx-related proteins playing roles in the intracellular vectorial transport of proteins-the products of the yeast Vps1 locus and the dynamins.