Interferon-α (IFN-α) is a protein family controlled by altogether 26 different IFN-α genes. We have previously described an SspI polymorphism in the IFN A17 gene and an association between the SspI A2 allele and nasopharyngeal cancer. In this paper we present data on ethnic differences with respect to IFN A17 SspI allele frequencies. Thus the frequency of the SspI A1 allele was high in two different Chinese populations (51 and 48%, respectively) and much lower (11%) in Swedes. Intermediate values were found in African Blacks (32%), Indians (25%), Saamis (29%) and Finns (24%). The very pronounced differences between major ethnic groups make the IFN A17 SspI polymorphism a very informative anthropological marker system and suggest that it may be balanced and maintained by natural selecti