Five restriction fragment length polymorphisms in the human alpha/beta interferon (IFN-alpha/beta) gene region were identified, three with an IFN-alpha probe and two with an IFN-beta probe. Heterozygosities are 74% for IFN-alpha, 57% for IFN-beta, and 87% jointly, making IFN-alpha/beta genes excellent markers for the short arm of chromosome 9. The pedigrees of about 25 families of Caucasian background were studied. Segregation analysis disclosed the occurrence of 12 of 32 possible haplotypes. No recombinant was found between IFN-alpha and -beta genetic markers; linkage disequilibrium within the IFN-alpha markers is of a similar order of magnitude as that between the IFN-alpha and -beta markers. The IFN-alpha and -beta genes might cluster within several hundred kilobases. In two parents, the IFN-beta gene is duplicated; the duplications segregate regularly. They are of independent, probably recent, origin. Therefore, some degree of multiplicity might even be found for IFN-beta, at least in some individuals.