Serum IgD concentrations were measured in normal Caucasian blood donors homozygous for either the Gm(a) Gm(g) or the Gm(f) Gm(b) linkage groups. IgD levels were measured by hemagglutination inhibition, single radial diffusion, and solid phase radioimmunoassay. The Gm(f) Gm(b) group had a significantly lower serum IgD level than did the Gm(a) Gm(g) group by all three methods of quantitation. The difference between the two groups was due primarily to the high frequency of low serum IgD levels among Gm(f) Gm(b) homozygotes. Although no genetic markers are available in IgD, the correlation with allotype markers of the IgG class indicates close linkage between IgD and IgG heavy chain C-region genes.