HUMAN fibroblast interferon is a single monomeric gly-coprotein1, and presumably a single structural gene codes for the human interferon polypeptide. It was originally reported that chromosomes 2 and 5 were necessary together for human interferon production2. More recent evidence suggests that both chromosomes 2 and 5 carry separate human interferon genes3,4. However, we have found, in two separate series of human-mouse somatic cell hybrid cell lines and their diphtheria toxin-resistant subclones5,6, that human interferon production segregated from chromosome 5 and, furthermore, did not correlate with the presence of chromosomes 2 and 5 together7. Furthermore, using 29 hybrid clones (13 independent primary hybrids and 16 subclones) we could exclude chromosomes 1-8, 10-22 and X as candidates for the localisation of a single gene coding for human interferon on the basis of discordance of more than three clones for the segregation of human interferon production and the presence of a particular chromosome. However, there was a good correlation between the production of human interferon and the presence of chromosome 9. Here we demonstrate this directly.