A presumed monospecific isoantibody to a β-lipoprotein antigen was found by the passive hemagglutination technique in the serum of a young man suffering from hemophilia A, who had received multiple blood transfusions. The antigen detected by this antiserum (HSH) occurred in 54.67% of 514 sera from Norwegian blood donors. A negative association between this antigen and the Ag(x) antigen was found. No significant association was found with age, or with blood groups ABO or Rh (D). Family studies revealed that the antigen is inherited as an autosomal dominant trait