AbstractThe human gene for mature interferon‐α1 (IFN‐α1) was inserted in a new transcription‐translation fusion vector system based on the expression and secretion signals of the gene for typeAstreptococcal pyrogenic exotoxin,speA. As deduced from the known nucleotide sequences of the component elements, the encoded IFN‐α1 was a fusion protein carrying an N‐terminal extension of 17 amino acids. When inserted in appropriate vectors capable of replication inEscherichia coli, Bacillus subtilisandStreptococcus sanguis, this expression configuration directed the synthesis of antiviral activity in all 3 organisms, as judged by the cythopathic effect inhibition assay of Vesicular Stomatitis Virus. InE. coliJM101, IFN activity was found mainly in the cytoplasmic protein fraction whereas in the gram‐positive hosts, it was completely secreted into the