Among the various proteins which are induced when human cells are treated with interferon, a predominant protein of unknown function, with molecular mass 56 kDa, has been observed. With the aim of exploring the molecular basis of the regulation of this protein and of its mRNA, in order to understand its biological function and its possible contribution to the various antiviral and non‐antiviral actions exerted by interferons, we cloned a full‐length cDNA copy of the 1.8 – 1.9 × 10³‐base 56‐kDa‐protein mRNA and determined its sequence. The cDNA contains 1662 nucleotides derived from the 56‐kDa‐protein mRNA, including a poly(A) tail of 20 residues. Primer extension experiments indicate that the 5′ end of this cDNA clone is probably located only 13 nucleotides downstream of the actual cap site of the 56‐kDa‐protein mRNA. It consists of a 64‐nucleotide‐long 5′‐non‐coding segment, a coding segment of 1434 nucleotides terminated by a TAG triplet and a 141‐nucleotide 3′‐non‐coding segment. The encoded protein of 478 amino acid residues has a molecular mass of 55335 Da and a single potential site for N‐glycosylation. The protein contains an excess of basic amino acids and most of them are localized in the carboxy‐terminal half of the molecule. A single [³⁵S]methoinine‐labeled 56‐kDa protein was obtained using an SP64 construction to allow the cell‐free transcription and translation of the cloned cDNA. Microinjection of this labeled protein in Xenopus oocytes indicates that the 56‐kDa protein is cytoplasmic.