A fraction enriched in interferon (IFN) mRNA was prepared from mouse C243-3 induced cells and was used for the construction of a cDNA library. Two plasmids were obtained after screening by differential colony hybridization and IFN mRNA hybridization-selection and translation. The nucleotide sequences of the cDNA inserts revealed that both were partial copies of IFN-beta mRNA. The cDNA 861 corresponds to the entire 3' nontranslated region of the mRNA while the cDNA 2939 consists of rearranged translated regions of IFN mRNA. A mechanism for the rearrangement events during cDNA synthesis is proposed. A chromosomal DNA fragment hybridizing to cDNA 2939 was identified by screening a mouse genomic library.