By means of quantitative agglutination, the results of interactions of A subgroup and variant genes with the B gene, as well as interactions of the Bx allele with A1, A2 and O genes in their heterozygous combination, are demonstrated. In the majority of interactions, competition occurs according to the rule that the allele producing the more active enzyme reduces the original activity of the enzyme produced by the relatively weaker allele. That the opposite result of interaction can occur also is shown in examples of Bx variants, when in A1Bx and A2Bx combinations complementation and a partial or nearly total normalization of the phenotypic expression of B antigen on red cells and in saliva happen. The phenomenon of competition, but even more so, the phenomenon of complementation, may have a broader importance than only for the ABO blood group system.