Serum hemagglutination-inhibition (HI) antibodies to parainfluenza type 1 virus and histocompatibility types were determined in 46 multiple sclerosis (MS) patients, 20 optic neuritis patients, and 54 control subjects. No significant differences in antibody titers were found when the MS, optic neuritis, and control groups were compared, nor when subjects bearing the HL-A3 histocompatibility antigen were compared with non-HL-A3 bearers. A significantly increased frequency of HLA3 was found among MS patients (42.9%) as compared with controls (23.0%) and optic neuritis patients (20.0%).