Two recombinant cosmids containing three complete murine interferon-alpha-encoding genes (Mu IFN-alpha) have been isolated from a mouse cosmid library. The cluster organization of these genes has been determined. A new Mu IFN-alpha gene (Mu IFN-alpha 11) has been isolated and studied with respect to its structure and inducible transcription pattern. The nucleotide and deduced amino acid sequences of the Mu IFN-alpha 11 gene, as compared to the two other IFN-alpha 7 and IFN-alpha 8 genes, show that, albeit highly homologous, these genes are all different. The transient expression of the three genes gave rise to proteins showing antiviral properties which were neutralized with murine anti-IFN-alpha antibodies. The transcription of the Mu IFN-alpha genes was studied in two uninduced or Newcastle-disease-virus-induced murine cell types. Mu IFN-alpha 11, as well as alpha 7 and alpha 8, are not expressed in L929 nor in C243 cells upon viral induction and therefore constitute an interesting model to study Mu IFN-alpha gene repression.