A hybrid plasmid, TpIF319, has been shown to contain the sequence for human fibroblast interferon mRNA [Taniguchi, T., Sakai, M., Fujii-Kuriyama, Y., Muramatsu, M., Kobayashi, S. & Sudo, T. (1979) Proc. Jpn. Acad. 55, Ser. B, 464-469]. This conclusion was confirmed by a hybridization-translation assay, using rabbit globin mRNA and its cDNA-containing plasmid as a control. Plasmid TpIF319 was used as probe and another recombinant plasmid, TpIF319-13, whose cDNA insert consists of about 800 base pairs, was isolated. Nucleotide sequence analysis of the cDNA revealed that the DNA in fact codes for human fibroblast interferon.