New variants of the ABOH blood group system are described which are similar to the ‘Bombay’ phenotype, but differ from it by Le(a—b + ) erythrocytes and secretion of ABH and Le^a and Le^b antigens. The erythrocytes of the proposita and members of her family are group O, negative with anti-H ; agglutinins if the ‘Bombay’ type, active also at 37°C, but weakened owing to interference by the secreted antigens, were found in the sera. The genetic background of the observed phenotypes is discussed. It appears likely that the atypical blood groups are due to a pair of recessive genes at a locus designated as Zjz and responsible for biosynthesis of H antigen in erythrocytes. The Zjz locus very probably belongs to the operator/ regulator gene complex for the structural gene H, and has a position similar that of the Yjy pair in relation to A gene. The homozygous combination zz causes suppression (modification) of the H phenotype in erythrocytes. As a result, the expression of the blood groups, as determined by the structural gene A or JB, is also suppressed. It is suggested that these variants (phenotypes) be given the symbols O(Hm), O^B(Hm), O^AB(Hm) the genetic information O, A, B or AB can be demonstrated only indirectly, and symbols A(Hm) and B(Hm) if the phenotype can be demonstrated by specific agglutination reactions.