A human leukocyte interferon cDNA was enzymatically synthesized, inserted into the vector pBR322, and cloned in Escherichia coli. The DNA sequence codes for a 23-amino acid signal peptide followed by an interferon polypeptide of 165 amino acids. An expression plasmid was constructed which permits the synthesis in E. coli of 2.5 x 10(8) units of interferon per litre of culture. This LeIF protected squirrel monkeys from lethal encephalomyocarditis virus infection.