A clone has been identified which contains the genetic information specifying human fibroblast interferon (beta 1) and which was derived from a messenger RNA population. On the basis of the nucleotide sequence the complete amino acid sequence (166 residues) of human fibroblast interferon was deduced. It is preceded by a 21 amino acids long signal sequence. Also, a genomic clone was isolated and characterized in detail; on this basis we can conclude that the interferon beta 1 gene does not contain introns. The nucleotide sequence of the putative promoter region was deduced and compared with other eukaryotic promoters. So far, no evidence has been obtained for other human interferons of the fibroblast type. The interferon beta 1 was expressed in the mature form of Escherichia coli without glycosylation. Fully processed interferon was obtained by transfection of monkey CV1-cells with an SV40-derivative, in which the interferon gene replaced the major structural protein VP1.