Studies have been carried out on polymorphism of adenosine deaminase in 36 Southern African populations comprising more than 3000 individuals. The common variant allele ADA2 has been found to attain polymorphic frequencies only in those populations descended from non-indigenous (i.e. non-Negro and non-Khoisan) groups. Its presence in certain other populations at low frequencies could be ascribed to small-scale Caucasoid admixture. A deficiency of the enzyme is found in certain members of the !Kung division of the San ('Bushman'). The low levels of enzyme activity are not associated with severe combined immunodeficiency and the gene which determines them appears to be polymorphic in the !Kung and possibly in some other San populations as well as possibly in Negro populations which have received substantial contributions of San genes.