A haplotype marker consisting of three biallelic restriction fragment length polymorphism (RFLP) loci from the VH-2 variable gene family was examined in 124 families with sibling pairs concordant for multiple sclerosis, 178 unrelated patients and 159 unaffected controls to investigate the role of the immunoglobulin heavy chain gene cluster in susceptibility to multiple sclerosis. Evidence for linkage was assessed using the affected sibling pair method of identity by descent, modified to allow for haplotype sharing on a probabilistic basis in families where haplotypes could not be assigned with certainty. The estimated probabilities of affected siblings sharing 0, 1 or 2 haplotypes were Z0 = 0.20, Z1 = 0.45, Z2 = 0.35. This deviation from the expected sharing probabilities of Z0 = 0.25, Z1 = 0.5, Z2 = 0.25 provides evidence for weak linkage (P < 0.05; equivalent to a lod score of 0.84); however, no significant allelic or haplotypic association was observed. Linkage without a population association suggests that a gene encoded on 14q confers susceptibility to multiple sclerosis, although this is not any of the existing VH-2 polymorphisms.