Data from the literature and our files on ABO, Rh and MN blood systems were analyzed to test their expected independent joint distribution. Both kind of data showed a highly significant interaction among these systems which cannot be mainly explained by technical or clerical errors. The frequency of A2 allele was higher in Rh (-) individuals, the cDe haplotype associates to the M allele, and B group showed a higher frequency of heterozygous MN and a lower one of homozygous NN. Several other distortions were heterogenously distributed among samples. The heterozygous state of one system seemed to interact with the zygotic state of the other ones. These systems are very probably interacting selectively. This kind of interaction could explain their maintenance at the polymorphic level in human populations.