Hm phenotype represents a dissociation between a normal salivary expression of H substance and a very weakened expression of the antigen on red blood cells. Genetic analysis of the reported family reveals a dominant inheritance: Some members (Marie K..., Francette, Carmen) present a phenotype marked by a normal H enzyme but a deficient H antigen in erythrocyte membrane. Others Alice, Mathilde) have no expression of A1 antigen due to H substrate deficiency. H substance in salivary secretion is normal. In the other branch of this pedigree without consanguinity, Herbert presents an H substance deficiency, though quite different, as A1 antigen is expressed. In this family, Hm phenotype can be explained, without resorting to a Zm allele, by the expression of an exceptional allele at the H locus (like Am is an ABO allele). This hypothesis supports the possible polymorphism of H locus.