Transfer of a 36-kilobase piece of human DNA containing the beta-interferon (IFN-beta) gene into mouse Ltk-cells leads to transient expression of human interferon even without an exogenous inducer. A low level of human interferon expression is also found in most stable clones containing the transferred DNA. With double-stranded RNA or Newcastle disease virus (NDV) as inducer, human interferon expression is greatly increased. The induced transcript is identical to normal human IFN-beta mRNA. Neighbouring genes contained on the transferred DNA are co-induced but are not essential for the production of human interferon in mouse L cells.