Antibody titers to measles, cytomegalovirus, and herpesviruses 1 and 2 were compared for siblings of multiple sclerosis (MS) patients sharing two, one, and no histocompatibility antigen haplotypes with the case. Significant differences were observed only for measles. Titers were significantly lower in siblings sharing no haplotypes with the case. Within case-sibling pairs, the presence of HLA-A-A3 and/or B7 affected measles antibody titers more than the presence of MS. These findings suggest that the immune response to measles in these sibships is influenced by the presence of HLA-A-A3 and/or B7 as well as another familial factor.