Thirty families in whom at least one member had asthma were typed for five new B-lymphocyte specificities, as well as the antigens of the A and B loci of HLA. The B-cell reactions were consistent with the concept that they were determined by genes linked to the HLA-A and HLA-B loci. Between 22 HLA-identical siblings and 16 two-haplotype different siblings, a significant difference in concordance of reactions for the B-cell groups was noted. The data obtained were most compatible with B-groups 1 and 2 being determined by one locus and B-groups 3, 4, and 5 by another locus. The superhaplotypes of HLA-A, HL-B, and B-cell loci 1 and 2 were then determined. The most striking linkages noted were for the second B-cell locus and the HLA-B locus: B-cell group 3 with HLA-B12, B-cell group 4 with HLA-B7, and B-cell group 5 with HLA-B8, B17, and Bw35.