The binding of human interferons to their binding site(s) was measured by the amount of radioiodinated human beta interferon (HuIFNbeta) displaceable by unlabeled human beta, alpha, and gamma interferon (HuIFNbeta, alpha, and gamma). By this approach, HuIFNbeta and HuIFNalpha were found to interact with specific binding sites in cell membranes derived from human cells and mouse-human cell hybrids containing chromosome 21 as their only human chromosome. Specific binding was not observed with cell membranes derived from parental mouse cells or from mouse-human cell hybrids in subsequent generations that have lost human chromosome 21. Although the chromosome 21-positive mouse-human cell hybrids are sensitive to the antiviral effects of HuIFNbeta and HuIFNalpha, they are found to be insensitive to the antiviral effect of HuIFNgamma and to lack specific HuIFNgamma binding sites. These results suggest that the HuIFNbeta and HuIFNalpha but not HuIFNgamma binding sites are coded for by genes located on chromosome 21. The lack of a chromosome 21 gene dosage effect on the inducibility of the antiviral state by HuIFNgamma is consistent with this hypothesis.