Interferon genes are usually only expressed after induction. In the accompanying paper we have shown that the accumulation of mRNA after viral induction is due to activation of transcription, rather than to reduction of turnover, and that the regulation of the alpha-interferon (IFN-alpha) gene is mediated by a segment of 5'-flanking region of not more than 700 base pairs (bp). To delineate the sequences required for induction, a set of 5' deletion mutants of the human IFN-alpha 1 gene was constructed and the expression of the truncated genes in mouse L cells was monitored after viral or mock infection. We report that not more than 117 bp of 5'-flanking sequence were required for induced expression of the gene. A purine-rich sequence of 42 bp located immediately downstream of position -117 is highly conserved in all known human alpha-interferon genes.