We have used restriction fragment length polymorphism (RFLP) analysis to investigate the immunoglobulin constant heavy chain (IgCH) loci and the associated locus Dl4Sl, of Caucasoids from South East England and South-West. West Germany, Haplotypes were determined using probes to the Ig heavy chain switch loci S mu and S alpha 1, the IgC gamma 3 and IgC gamma 2 loci as well as the Dl4Sl locus which is 3' of the IgCH loci. The 6.3:1.7 kilobase (kb) Bst EII C gamma 3-C gamma 2 haplotype was the most prevalent in the population from South-East England (frequency 0.364), whilst the 2.3:3.7 kb C gamma 3-C gamma 2 haplotype was the major haplotype in the German population (frequency 0.400). With one exception, the major haplotypes of these two populations differed from the ones previously published for a Caucasoid population from California. This suggests that there may be a major ancestral IgCH haplotype which has been maintained in the population, whilst other haplotypes tend to be specific for a particular group of Caucasoids.