In a case of disputed paternity mother and child showed a genetic incompatibility in the Kidd system of the erythrocytes: mother Jk(a-b+), child Jk (a+b-). For the first time the RFLP technique solved this problem, using four different Jeffreys' highly polymorphic single locus DNA probes MS 1, MS 31, g 3 and MS 43. We found that mother and child possessed one identical gene observed in each of the four probes. The confidence of somatic genuineness of this mother-child pair is very strong: p = 10(9). In addition, the above-mentioned four DNA probes showed identical results in 12 cases of paternity exclusion and 18 nonexclusions found by 23 conventional hereditary systems. Furthermore, in five cases of bone marrow transplantation success of this therapy was proven by the RFLP technique, which differentiates between recipient genes and donor genes. Our investigations on hereditary so far comprise five families with 10 children and one family over four generations.