The human leukocyte interferon-alpha L (IFN-alpha L) appears to be a pseudogene because it contains a termination codon in the DNA coding for the precursor peptide (signal peptide), but the remainder of the gene codes for an interferon protein of normal length. To determine if this gene codes for an active interferon molecule, an expression plasmid was constructed for IFN-alpha L that was produced in Escherichia coli. The IFN-alpha L is active on human and bovine cells, and exhibits a trace of activity on mouse cells.