In a case of disputed paternity genetic incompatibility was observed in the ABO blood group system between mother (O) and child (AB). From biostatistical evaluation of 21 genetic markers, including HLA phenotypes, a high value of probability for paternity, maternity and parentage was found between the child, the child's mother, the accused man and his mother. Substitution of the newborn was thereby excluded. The serostatistical evaluation of maternity and paternity could be supported by anthropological opinion. In serological investigations of the ABH system the A and B antigens of the 'A2B' cells were found to be abnormal in the child, the child's father, and the father's mother: the A was weaker than normal A1 but stronger than normal A2; the B was found to be abnormal which in addition was documented through occurrence of irregular anti-B antibodies in the child. In comparison to normal controls (A1, A2, B, A1B, A2B) diminished activity of alpha-D-N-acetylgalactosamine and alpha-D-galactosyltransferase were observed in the three 'A2B' propositi. These facts confirmed the existence of a cis-AB gene in the Lam. family which the child inherited from her father and the child's father from his mother. Assuming a population frequency of 1.1 X 10(-5) for the cis-AB gene, the probability for paternity was calculated from all genetic markers to be W = 99.9999985%.