Recombinant plasmids for intracellular synthesis of mature porcine interferon alpha 1 (IFN-alpha 1) in Escherichia coli were constructed. High amounts of antiviral activity were obtained [up to 4 x 10(5) international units (IU) per ml of bacterial culture]. Recombinant porcine IFN-alpha 1 (rIFN-alpha 1) was purified to homogeneity by monoclonal antibody immunoaffinity and was found to have the expected Mr (17.5K) and N-terminal sequence (except for the apparent lack of an N-terminal methionine). Its specific antiviral activity was 5 x 10(7) to 10 x 10(7) IU/mg MDBK cells. In vitro biological properties of this purified rIFN-alpha 1 were compared to those of virus-induced porcine leukocyte interferon: the two interferons shared similar antigenic determinants and had the same ability to induce a cytocidal effect on primary cultures of pig kidney epithelial cells. However, rIFN-alpha 1 was at least six times more active in inducing an antiviral state on homologous porcine cells. These properties are discussed in the light of a possible in vivo use of the purified recombinant molecule.